Single stranded helical chains of C-H?π interactions further connected by halogen-halogen interactions of type i to construct supramolecular structure of (E)-5-(diethylamino)-2-[(4-iodophenylimino)methyl]phenol compound

Article abstract of DOI:10.1016/j.saa.2012.03.058

In this study, (E)-5-(diethylamino)-2-[(4-iodophenylimino)methyl]phenol compound was investigated from the point of stacking interactions assembling the supramolecular network, conformational isomerism and tautomerism. For this purpose, X-ray diffraction, FT-IR and UV/Vis spectroscopic techniques were used, giving the following structural and spectroscopic properties of the compound: The title compound has two conformers (anti and eclipsed) in the crystal structure resulting from rotation about C-N single bond of ethyl group. Both conformers prefer enol form in the solid state, adopting E configuration about the CN double bond. The supramolecular architecture of the compound is constructed by two non-covalent interactions as C-H?π and halogen-halogen interactions. The repetition of C-H?π interactions is resulted in a single-stranded helical structure. The helical structures are further connected by C-I?I-C interactions of Type I to construct the two dimensional supramolecular network defined as (6,3)-net in Wells nomenclature. The title compound adopts both enol and keto forms in EtOH (a polar and protic solvent) while enol form is preferred in the solid state.

Full text of DOI:10.1016/j.saa.2012.03.058

Spectrochimica Acta Part A 94 (2012) 200–204
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Single stranded helical chains of C–H· · ·␲ interactions further connected by
halogen–halogen interactions of type I to construct supramolecular structure of
(E)-5-(diethylamino)-2-[(4-iodophenylimino)methyl]phenol compound
Gökhan Kas¸ tas¸ a,∗, C¸ ig˘dem Albayrakb, Mustafa Odabas¸ og˘luc, René Frankd
a Ondokuz Mayıs University, Faculty of Arts and Sciences, Department of Physics, 55139, Samsun, Turkey
b Sinop University, Faculty of Education, Department of Science Education, 57000, Sinop, Turkey
c Pamukkale University, Chemical Technology Program, 20070, Kınıklı-Denizli, Turkey
d Leipzig University, Faculty of Chemistry and Mineralogy, D-04103, Leipzig, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
In this study, (E)-5-(diethylamino)-2-[(4-iodophenylimino)methyl]phenol compound was investigated
from the point of stacking interactions assembling the supramolecular network, conformational iso-
merism and tautomerism. For this purpose, X-ray diffraction, FT-IR and UV/Vis spectroscopic techniques
were used, giving the following structural and spectroscopic properties of the compound: The title com-
pound has two conformers (anti and eclipsed) in the crystal structure resulting from rotation about C–N
single bond of ethyl group. Both conformers prefer enol form in the solid state, adopting E configura-
tion about the C N double bond. The supramolecular architecture of the compound is constructed by
two non-covalent interactions as C–H· · ·␲ and halogen–halogen interactions. The repetition of C–H· · ·␲
interactions is resulted in a single-stranded helical structure. The helical structures are further connected
by C–I· · ·I–C interactions of Type I to construct the two dimensional supramolecular network defined as
(6,3)-net in Wells nomenclature. The title compound adopts both enol and keto forms in EtOH (a polar
and protic solvent) while enol form is preferred in the solid state.
Received 29 November 2011
Received in revised form 8 March 2012
Accepted 22 March 2012
Keywords:
Schiff base
Intramolecular hydrogen bond
Intermolecular hydrogen bond
Halogen–halogen interaction
Tautomerism
Helical chain
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
conformational isomerism and tautomerism. For this purpose,
molecular structure and spectroscopic properties of the com-
o-Hydroxy Schiff bases are of interest because of their ther-
mochromism and photochromism in the solid state, which can
involve reversible proton transfer from an oxygen atom to the
neighboring nitrogen atom. Intramolecular proton transfer mech-
anism for excited-state and ground-state is still investigated
intensively. Based on thermochromic and photochromic Schiff
bases, it was proposed that molecules exhibiting thermochromism
are planar while molecules exhibiting photochromism are non-
planar [1,2]. Photochromic compounds are used as optical switches
and optical memories, variable electrical current, ion transport
through membranes [3]. In addition, they have widespread usage
as ligands in the field of coordination chemistry [4] as well as in
diverse fields of chemistry and biochemistry owing to their biolog-
ical activities [5]. o-Hydroxy Schiff bases can exist in two tautomeric
structures as enol and keto forms in the solid state [6,7].
pound were characterized by X-ray diffraction, FT-IR and UV–vis
spectroscopic techniques.
2. Experimental methods
2.1. Synthesis
o-Hydroxy Schiff bases are prepared by the condensation reac-
tion of primary amines with carbonyl compounds [8–10]. The
same method was used in the synthesis of (E)-5-(diethylamino)-2-
[(4-iodophenylimino)methyl]phenol compound. More specifically,
the mixture of 5-(diethylamino)-2-hydroxybenzaldehyde (0.5 g,
2.59 mmol, 20 ml ethanol) and 4-iodoaniline (0.57 g, 2.59 mmol,
20 ml ethanol) were stirred for 1 h under reflux. The crystals of the
title compound suitable for X-ray analysis were obtained by slow
evaporation from acetone (yield 76%; m.p. 384–385 K).
In this study, (E)-5-(diethylamino)-2-[(4-iodophenylimi-
no)methyl]phenol compound was studied focusing on stack-
ing interactions assembling the supramolecular network,
2.2. Instrumentation
The melting point was determined by StuartMP30 appa-
ratus. FT-IR spectrum of the compound was recorded in the
4000–400 cm−1 region with a Bruker 2000 spectrometer using KBr
∗
Corresponding author. Tel.: +90 3623121919; fax: +90 3624576081.
E-mail address: gkastas@omu.edu.tr (G. Kas¸ tas¸ ).
1386-1425/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2012.03.058

G. Kas¸tas¸ et al. / Spectrochimica Acta Part A 94 (2012) 200–204
201
Fig. 1. A view of the title compound, showing anti and eclipsed conformers. H atoms of methylene groups were omitted for clarity.
pellet. UV–vis absorption spectra were recorded on a Thermo sci-
entific BioGenesis UV–vis spectrometer using a 1 cm path length
of the cell. Diffraction experiment was carried out at 130 K
with an Xcalibur diffractometer using graphite monochromated
MoK␣ radiation. The structure was solved by direct methods
using SHELXS-97 [11]. The refinement was carried out by full-
matrix least-squares method. All non-hydrogen atoms were refined
anisotropically. The H atoms except those bonded to O atoms were
placed in geometrically idealized positions and refined by a riding
model with Uiso(H) = 1.2Ueq(C) and Uiso(Hmethyl) = 1.5Ueq(C). The
relevant crystal data, experimental conditions and final refinement
parameters can be obtained from Cambridge Structural Database
(CCDC: 801849). WinGX [12], ORTEP-3 for Windows [13] and MER-
CURY [14] softwares were used for molecular drawings and other
materials.
to two conformers in the crystal structure. The repetition of
intermolecular C17i–H17Bi· · ·Cg1ii (i: 5 − x, 1.5 + y, 2 − z; ii: 2 − x,
1.5 + y, 1 − z) and C34ii–H34Aii···Cg2iii (iii: x, 1 + y, z) interactions
is resulted in C22(18) chains (Fig. 2). Interestingly, the extension of
these chains through [0 1 0] is resulted in a single-stranded helical
˚
structure with a pitch distance of 10.265 A between iodide atoms
(Fig. 2). One another interesting property is that single stranded
3. Results and discussion
3.1. Structure determination
There are two crystallographically independent molecules in the
asymmetric unit, which correspond to anti and eclipsed conformers
of the title compound (Fig. 1). The two conformers are resulted from
rotation about C16–N2 single bond of ethyl group. In conformer
I (anti conformer), the ethyl groups are positioned in the oppo-
site directions. On the other hand, the ethyl groups in conformer II
(eclipsed conformer) are in the same directions. The torsion angle
C4–N2–C16–C17 = 92.1(3)◦ in anti conformer while its correspond-
ing value in eclipsed conformer is 78.5(4)◦. The characteristic bond
distances and angles related to two conformers are comparatively
given in Table 1. The compound prefers enol form in both conform-
ers. The C N double bond and C–O single bond distances compare
well with reported values for similar compounds [15,16]. Both con-
formers adopt E configuration about the C N double bond (Fig. 1).
The strong intramolecular interactions involving phenolic atoms
(O1, O2) and nitrogen atoms (N1, N3) constitute six-membered
rings defined as S(6) in Etter’s notation [17]. These hydrogen bonds
are characterized by O1· · ·N1 and O2· · ·N3 distances of 2.613(3) Å
˚
and 2.595(3) A (Table 1), being shorter than the sum of the van der
Waals’ radii for N and O [18]. It is known that there is a strong
correlation between the strength of the H-bond and the delocal-
ization of the system of conjugated double bonds, and the effect
is qualitatively interpreted by resonance-assisted hydrogen bond
˚
(RAHB) model [19]. The observed O· · ·N distances (2.613(3) A and
˚
˚
2.595(3) A) for the title compound are shorter than 2.656 A reported
for O–H· · ·N in the class of RAHB [20].
The supramolecular architecture of the title compound is
constructed by two non-covalent interactions as C–H· · ·␲ and
halogen–halogen interactions. The C–H· · ·␲ interactions take place
between methyl groups and phenyl rings (Cg). It should be noted
that C–H· · ·␲ interactions only involve the ethyl groups leading
Fig. 2. The formation of a single-stranded helical structure in the crystal packing
of the title compound by non-covalent C–H· · ·␲ interactions [Symmetry codes: (i)
5 − x, 1.5 + y, 2 − z; (ii) 2 − x, 1.5 + y, 1 − z; (iii) x, 1 + y, z].

202
G. Kas¸tas¸ et al. / Spectrochimica Acta Part A 94 (2012) 200–204
Table 1
Bond distances (Å), angles (◦) and non-covalent interactions between anti and eclipsed conformers of the title compound.
Bond distance
Anti
Bond angle
Anti
Torsion angle
Anti
Eclipsed
Eclipsed
Eclipsed
N2–C4
N2–C16
N2–C14
1.372(3) N4–C21
1.447(4) N4–C31
1.459(4) N4–C33
1.377(4) N2–C14–C15 112.4(3)
1.460(4) N2–C16–C17 112.5(3)
1.501(5) C14–N2–C16 115.9(2)
N4–C33–C34 109.2(4) C4–N2–C16–C17 92.1(3)
N4–C31–C32 114.8(3) C4–N2–C14–C15 96.0(3)
C21-N4–C31–C32
C21-N4–C33–C34
78.5(4)
-81.0(4)
-179.6(2)
-0.5(5)
-177.3(3)
-0.9(4)
176.9(3)
-158.5(3)
24.5(4)
C31–N4–C33 116.4(3) C1–C7–N1–C8
C21–N4–C33 122.2(3) C3–C4–N2–C14
C21–N4–C31 121.3(3) C3–C4–N2–C16
C20–C21–N4 121.3(3) O1–C2–C1–C7
C22–C21–N4 121.0(3) N1–C7–C1–C6
C24–N3–C25 120.9(2) C7–N1–C8–C9
C18–C19–O2 120.1(2) C7–N1–C8–C13
176.9(2) C18–C24–N3–C25
-15.7(4) C20–C21–N4–C33
170.2(3) C20–C21–N4–C31
C14–C15 1.515(4) C31–C32 1.522(5) C4–N2–C14
C16–C17 1.521(4) C33–C34 1.453(6) C4–N2–C16
C2–O1
N1–C7
N1–C8
C11–I1
121.9(2)
122.0(2)
121.1(3)
120.6(3)
118.5(2)
120.8(2)
1.360(3) C19–O2
1.294(4) N3–C24
1.411(3) N3–C25
2.093(3) C28–I2
1.352(3) C3–C4–N2
1.284(4) C5–C4–N2
1.411(3) C7–N1–C8
2.095(3) C1–C2–O1
0.9(4)
O2–C19–C18–C24
174.4(3) N3–C24–C18–C23
143.9(3) C24–N3–C25–C26
-39.7(4) C24–N3–C25–C30
aD–H· · ·A
d(D–H)
0.788
0.837
Cg
d(H· · ·A)
1.914
1.830
d(D· · ·A)
<D−H· · ·A
O1–H1· · ·N1
O1–H2· · ·N3
aX–H
2.613
147
2.595
151
d(H· · ·Cg)
d(X· · ·Cg)
<X−H· · ·Cg
C17–H17B
C34–H34A
aC–X1
Cg1iv
Cg2v
2.830
3.569
133
2.810
3.634
142
X2–C
I2–C28
X1· · ·X2
<C–X1· · ·X2
<X1· · ·X2–C
C11–I1
3.791
150
155
a
Symmetry codes: (iv) 3 + x, y, 1 + z; (v) 2 − x, −1/2 + y, 1 − z. Cg represents ring centroid. There are no s.u values in geometric parameters of non-covalent interactions since
most of H atoms were placed in geometrically idealized positions.
helical structures are connected by C–I···I–C interactions with
a two-dimensional network is obtained (Fig. S2), which is defined
as (6,3)-net in Wells nomenclature or 63 in Schläfli notation [22].
˚
I1···I2 distance of 3.7913(3) A (Fig. 3) to construct two dimen-
sional supramolecular structure of the compound. In general,
the intermolecular C–X1· · ·X2–C interactions (X = F, Cl, Br, I) are
classified into two types depending on the values of the angles
3.2. FT-IR spectrum
ꢀ1 = C–X1· · ·X2 and ꢀ2 = X1· · ·X2–C. The interactions with similar
The vibrational investigation of the title compound was car-
ried out at mid-IR region, mainly focusing on the characteristic
vibrations. The FT-IR spectrum of the compound is provided as sup-
plementary material (Fig. S3). The OH group has three vibrations
as stretching, in-plane bending and out-of-plane bending modes.
The absorption band in 1700–3000 cm−1 region is attributed
to the ꢁ(O–H) stretching vibration which broaden due to the
formation of strong intramolecular O–H· · ·N hydrogen bond in the
structure. Generally, the OH in-plane bending and out-of-plane
bending vibrations of phenols lie in the region 1300–1400 cm−1
and 517–710 cm−1, respectively [23]. For the title compound, the
∼
angles (ꢀ1 ꢀ2) are called Type I which can have two possible
=
geometries (cis or trans) ◦by definition◦(Fig. S1). On the other hand,
∼
∼
=
the condition of ꢀ1 180 and ꢀ2 90 (L-geometry) is satisfied for
=
the interactions of Type II [21]. In the title compound, the geometry
of halogen-halogen interactions is characterized by C11–I1· · ·I2
(ꢀ1) and C28–I2· · ·I1 (ꢀ2) angles of 149.46(8)◦ and 154.99(8)◦,
respectively. The trans geometry of this interaction corresponds to
halogen interaction of Type I. From topologic point of view, each
conformer can be considered as a node. In this sense, each node
is connected to three nodes forming a T-shaped geometry. Thus,
Fig. 3. Two dimensional supramolecular network of the title compound formed by halogen-halogen interactions between single stranded helical structures based on C–H· · ·␲
interactions. Symmetry codes are given in Table 1.

G. Kas¸tas¸ et al. / Spectrochimica Acta Part A 94 (2012) 200–204
203
band at 1343 cm−1 was assigned to OH in-plane bending vibra-
tion and the band at 786 cm−1 was assigned to OH out-of-plane
bending vibration. The absorption band located at 1623 cm−1
is attributed to ꢁ(C N) stretching vibration. The aromatic C–H
stretching, C–H in-plane bending and C–H out-of-plane bending
vibrations appear in 3000–3100 cm−1
,
1100–1500 cm−1 and
800–1000 cm−1 frequency ranges, respectively [23]. The absorp-
tion band at 3056 cm−1 corresponds to the aromatic CH stretching
vibrations of the title compound. In addition, in-plane bending
and out-of-plane CH vibrations were observed at 1127 cm−1
and at 821 cm−1, respectively. The asymmetric and symmetric
stretching vibrations of the aliphatic CH2 and CH3 groups were
observed at 2971, 2933 and 2889 cm−1. The bending vibrations of
these groups were observed at 1448, 1422 cm−1 and 1381 cm−1
while rocking modes were observed at 1014 and 701 cm−1. The
C–O stretching vibrations in phenols are expected to appear as
a strongest band in 1300–1100 cm−1 frequency ranges [23]. The
absorption band observed at 1195 cm−1 for the title compound
was assigned to C–O stretching mode of vibration. The absorption
bands observed at 1600–1400 cm−1 are due to CC stretching vibra-
tions of the aromatic compounds [23]. The CC stretching modes
of aromatic rings of the title compound were observed at 1597,
1568, and 1516, 1479 cm−1, which agree well with the literature
data.
Fig. 4. The solvent effect on UV–vis spectra of the title compound in (– · –) DMSO,
(–) EtOH, (– – –) THF, (. . .) CHCl3, (– ·· –) CH3CN, (– – ·) benzene.
EtOH (Fig. 4), which was not observed in case of DMSO, THF, CH3CN,
benzene and CHCl3.
The previous computational and experimental studies show
that the new absorption band above 400 nm belongs to the
keto form of o-hydroxy Schiff bases [27–29]. Referring to previ-
ous studies it was concluded that the title compound exists in
both enol and keto form in EtOH while it prefers enol form in
the solid state. The keto form is important in the solution and
stabilized by the polar solvents through solute–solvent interac-
tions. Fig. 5 shows the possible structures of compounds in the
solution.
The substituents increasing the imino nitrogen basicity and
the hydroxyl acidity ease the formation of keto form [30]. N,N-
diethylamino group is an electron-donating group and increases
nitrogen basicity since it is in para position with respect to the imino
group (Fig. 5). N,N-diethylamino group stabilizes zwitter ionic form
and eases the formation of keto form with resonance. If it depended
on the electron-donating N,N-diethylamino group, keto form would
have observed in all solvents. This shows that the formation of keto
form is not only depended on electron-donating N,N-diethylamino
group. The type of solvent is also important. Moreover, the forma-
tion of keto form cannot be explained only by the polarity of solvent.
If it depended on the polarity of solvent, keto form would have
observed in DMSO. The fact that keto structure did not formed in
DMSO shows that the solvent polarity is not effective on the proton
transfer and instead it depends on the H-donor–acceptor property
of solvent. The title compound adopts both enol and keto forms in
EtOH which is a polar and protic solvent. Previous computational
3.3. UV–vis absorption spectra
The proton transfer equilibrium in o-hydroxy Schiff bases
depends on the substituent and solvent. Previous studies show that
the Schiff bases derived from salicylaldehyde generally prefer the
enol form in both polar and apolar solvents while some Schiff bases
of 2-hydroxynaphtaldehyde exist in keto form [24,25]. The studies
of Dziembowska et al. on Schiff bases derived from substituted sal-
icylaldehyde indicated that the effect of solvent on taoutomeric
equilibrium is especially depended on electronic structure of par-
ticular Schiff bases [26]. If there is an electron donating substituent
in para position with respect to imino group, the formation of H-
bond between the protic solvent and oxygen atom stabilizes the
keto form. Thus, the tendency of the equilibrium to shift to keto
form becomes easier [26].
In this study, UV–vis spectra of the title compound in various
organic solvents (EtOH, DMSO, THF, CH3CN, benzene and CHCl3)
were recorded within 200–500 nm range (Fig. 4) in order to inves-
tigate the effect of substituent and solvent. The results obtained for
the title compound correspond to same electronic transitions in
all solvents except EtOH. The absorption bands at 360 nm (in THF,
CH3CN, benzene and CHCl3) and 366 nm (in DMSO) are attributed
to ␲ → ␲* transition of enol form. However, a new absorption band
at 412 nm was observed in the spectrum of the title compound in
Fig. 5. The possible structures of the title compound.

204
G. Kas¸tas¸ et al. / Spectrochimica Acta Part A 94 (2012) 200–204
studies also proved that protic solvents decrease activation energy
and ease proton transfer [31,32].
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(E)-5-(diethylamino)-2-[(4-iodophenylimino)methyl]phenol
compound has two conformers (anti and eclipsed) in the crystal
structure resulting from rotation about C–N single bond of ethyl
group. Both conformers prefer enol form in the solid state, adopting
E configuration about the C N double bond. The two dimensional
supramolecular architecture of the compound is constructed by
non-covalent C–H· · ·␲ and C–I· · ·I–C interactions. The repetition of
C–H· · ·␲ interactions forms a single-stranded helical structure. The
helical structures are further connected by C–I· · ·I–C interactions of
Type I to construct the two dimensional supramolecular network
defined as (6,3)-net in Wells nomenclature. The title compound
adopts both enol and keto forms in EtOH (a polar and protic
solvent) while enol form is preferred in the solid state.
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Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.saa.2012.03.058.
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