Crystal structure and feasibility of intramolecular proton transfer reaction of salicylaldazine

Article abstract of DOI:10.1016/j.molstruc.2004.02.030

The work describes characterization of salicylaldazine by IR, NMR and single crystal X-ray diffraction studies. The compound crystallizes in the monoclinic crystal system with the space group P2₁/n, a=8.554(3), b=6.338(2), c=11.864(5)?, and β=107.89(2)°. There are two strong intramolecular hydrogen bonds of the type O-H?N and two weak hydrogen bridges of the type C-H?O which help the molecules to pack in a layered structure. Semi-empirical calculations have been performed to predict theoretically the feasibility of intramolecular ground and/or excited state proton transfer reaction/s. Potential energy curves have been generated in the ground (S₀) and the lowest excited (S₁) singlet states to judge the feasibility of intramolecular single and double proton transfer reactions. Semi-empirical (AM1) calculations suggest that only the excited state intramolecular single proton transfer process is favored both thermodynamically and kinetically. No other prototropic processes are theoretically feasible.

Full text of DOI:10.1016/j.molstruc.2004.02.030

Journal of Molecular Structure 693 (2004) 161–165
www.elsevier.com/locate/molstruc
Crystal structure and feasibility of intramolecular proton
transfer reaction of salicylaldazine
a
Md. Mijanuddin , W.S. Sheldrick , H. Mayer-Figge , Mahammad Ali , Nitin Chattopadhyay
b
b
a
a,*
a
Department of Chemistry, University of Jadavpur, Jadavpur, Calcutta 700 032, India
b
Ruhr-Universitat Bochum Lehrstuhl fur Analytische Chemie, Bochum D-44780, Germany
Received 25 November 2003; revised 21 February 2004; accepted 24 February 2004
Abstract
The work describes characterization of salicylaldazine by IR, NMR and single crystal X-ray diffraction studies. The compound crystallizes
˚
in the monoclinic crystal system with the space group P2 =n; a ¼ 8:554ð3Þ, b ¼ 6:338ð2Þ, c ¼ 11:864ð5Þ A, and b ¼ 107:89ð2Þ8: There are
1
two strong intramolecular hydrogen bonds of the type O–H· · ·N and two weak hydrogen bridges of the type C–H· · ·O which help the
molecules to pack in a layered structure. Semi-empirical calculations have been performed to predict theoretically the feasibility of
intramolecular ground and/or excited state proton transfer reaction/s. Potential energy curves have been generated in the ground (S
0
) and the
lowest excited (S ) singlet states to judge the feasibility of intramolecular single and double proton transfer reactions. Semi-empirical (AM1)
1
calculations suggest that only the excited state intramolecular single proton transfer process is favored both thermodynamically and
kinetically. No other prototropic processes are theoretically feasible.
q 2004 Elsevier B.V. All rights reserved.
Keywords: Salicylaldazine; Intramolecular proton transfer; Hydrogen bonding; Potential energy curve
1
. Introduction
Osborn and Youinou showed that the ligand 3,6-di-
(2-pyridyl)pyradazine (dppn) forms 2 £ 2 grid arrays with
Self-assembly by H-bonding and van der Waals’
tetrahedral metal ions [7], a concept which Baxter et al.
elegantly extended to create higher order N £ N grid arrays
[8]. In contrast to dppn with its rigid central ring, aldazines
have the freedom to rotate about their N–N central bond and
so permit formation of architectures other than the grid
arrays. An interesting feature of the ligand, salicylaldazine,
is that due to the presence of OH groups close to the azine
nitrogens, which exhibit a trans orientation, there are ample
possibilities for intramolecular hydrogen bonding. There is
also the possibility that intramolecular proton transfer (IPT)
might take place between these two groups. A cursory look
at the symmetrical molecular structure would suggest the
possibility of intramolecular single as well as double proton
transfer (DPT) reactions. Hence, characterization of these
ligands from spectroscopic and X-ray diffraction studies is
important from structural point of view. The aim of this
article is to study the structural aspects of salicylaldazine
and also to find out theoretically the feasibility of the ground
state and the excited state intramolecular prototropic
processes.
interaction is a very important process for the formation
of biological architecture [1–3]. Hydrogen bonds are so
widespread in chemistry and biology and have so many
structural and mechanistic consequences that they have
been rapidly outpaced by observations concerning what
they can do [4,5]. These mechanisms are being developed as
efficient design tools in material science for organizing
individual molecular motifs into highly ordered
supramolecules.
Aldazines are the ligands obtained by condensation of
aromatic aldehydes with hydrazines. These compounds are
known to behave as tridentate chelating agents, in which
either of the two azine nitrogens coordinates along with the
two pyridine nitrogens [6]. The molecules are coplanar
because of the high degree of conjugation and therefore
coordinate along an edge.
*
Corresponding author. Tel.: þ91-33483-4133; fax: þ91-33414-6266.
E-mail addresses: pcnitin@yahoo.com (N. Chattopadhyay);
m_ali2062@yahoo.com (M. Ali).
0022-2860/$ - see front matter q 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2004.02.030

162
Md. Mijanuddin et al. / Journal of Molecular Structure 693 (2004) 161–165
2
. Experimental
Starting materials for the synthesis of salicylaldazine,
Table 1
12 2 2
Crystal data, structure solution and refinement parameters for C14H N O
Empirical formula
Formula weight
Color, habit
14 12 2 2
C H N O
namely, salicylaldehyde (Merck, India) and hydrazine
hydrate 80% GR (Loba, India) were used as received.
Solvents like methanol and tetrahydrofuran (Merck, India)
were of reagent grade and dried by standard methods.
Salicylaldazine was synthesized by condensation of hydra-
zine hydrate and salicylaldehyde in a 1:2 molar ratio in
methanol. The yellow powder-like product was filtered-off
and washed with methanol. The block-shaped single yellow
crystals for X-ray analysis were obtained from slow
evaporation of a tetrahydrofuran solution. FTIR (KBr)
120.13
Yellow
Crystal size/mm
Crystal system
Space group
0.66 £ 0.51 £ 0.61
Monoclinic
P2 =n
1
˚
Unit cell dimensions
a ¼ 8:554ð3Þ A, a ¼ 908;
b ¼ 6:338ð2Þ A˚ , b ¼ 107:89ð2Þ8;
˚
c ¼ 11:864ð5Þ A, g ¼ 908
3
˚
Volume (A )
612.0(4)
2
Z
2
3
)
Density (calc., Mg m
1.304
0.089
252
21
Absorption coefficient (mm
)
signals were consistent with the observations of Abo Ali [9].
1
Fð000Þ
H NMR (in CDCl , 300 MHz): d ¼ 6:95–7:05 (m, 2 H,
3
Diffractometer (Siemens P4)
Temperature (K)
arom.H), 7.26–7.42 (m, 2 H, arom.H), 8.70 (s, 1 H, aliph.
293(2)
1
H), 11.38 (s, 1 H, phenolic H). C NMR (in CDCl3,
3
u range (8) for data collection
Limiting indices
3.69–27.49
75 MHz): d ¼ 117:05 (C6, arom.), 117.17 (C2, arom.),
119.62 (C4, arom.), 132.45 (C3, arom.), 133.34 (C5, arom.),
159.69 (C7, arom.), 164.6 (C1, aliph.).
211 # h # 1; 21 # k # 8;
215 # l # 15
Reflections collected/unique
1934/1383, RðintÞ ¼ 0:0339
Completeness to theta ¼ 27.49
Refinement method
98.9%
2
Full-matrix least-squares on F
1383/0/89
2
.1. Crystal data collection and refinement
Data/restraints/parameters
2
Goodness-of-fit on F
1.052
Final R indices ðI . 2sðIÞÞ
R indices (all data)
R1 ¼ 0:0426; wR2 ¼ 0:1156
R2 ¼ 0:0564; wR2 ¼ 0:1243
0.06(3)
Intensity data for yellow salicylaldazine was collected for
934 reflections (1383 unique reflections) at 293(2) K on a
Siemens P4 diffractometer using graphite monochromatized
1
Extinction coefficient
˚
Largest diff. peak and hole (e A
23
)
0.232 and 20.139
˚
Mo Ka radiation (0.71073 A). The employed v–2u scan
mode was in the range 3:69 # u # 27:58: No decomposition
of the crystal during the data collection was noted. The
intensities were corrected for Lorentz and polarization
effects. The structures were solved by direct methods with
SHELXS-97 and non-hydrogen atoms refined anisotropically
The methods provide acceptable approximations to give
results quite close to the experimental findings [10–16]. The
reliability of the method of calculation (AM1-SCI) has
already been established through studies on different types
of molecular systems varying in their photophysical
properties. For the present calculations, we have used the
commercial package, HyperChem 5.01 (Hypercube Inc.,
Canada). The geometry of the molecule has been optimized
in the ground state using the AM1 method. AM1-SCI has
been adopted for the calculation of the excited state
energies. It is pertinent to mention here that present
calculations have been performed for the free molecule in
vacuum only and no specific interactions like hydrogen
bonding etc. have been taken into account.
2
1
by full matrix least-squares on F using SHELXS-97.
Refinements give R ¼ 0:043½I . 2sðIÞꢀ and wR ¼ 0:124
1
2
(all data) for salicylaldazine. Final refinement details are
given in Table 1. The maximum and minimum peaks in the
2
3
˚
final difference map were 0.232 and 20.139 e A
.
Crystallographic data for the compound has been deposited
with the Cambridge Crystallographic Data Centre (CCDC
No. 224827). Copies may be obtained free of charge on
application to the Director, CCDC, 12 Union Road, Cam-
bridge CB2 1EZ, UK (fax: int. Code þ 44(0)1223/336-033,
e-mail: deposit@chemcrys.cam.ac.uk).
2
.2. Theoretical calculations
3
. Results and discussion
Although, ab initio calculations involving extended basis
3
.1. Description of the structures
sets with extensive configuration interaction (CI) have been
successful in explaining structures, energetics and reacti-
vities of small molecules in different electronic states, such
reports are still limited in number for large molecular
systems. Semi-empirical molecular orbital methods have
already established their wide usefulness in this respect.
Salicylaldazine crystallizes in the monoclinic space
group P2 =n: Table 1 lists the crystallographic data and
1
refinement parameters and Table 2 presents selected bond
lengths and bond angles. The crystallographic parameters
are consistent with the reports of Xu et al. [17]. The
calculated values of the parameters for the optimized
structure are given in italics. The calculated values are in
1
˚
O–H and C–H distances of 0.82 and 0.93 A were employed by SHELX
for the calculation of the respective H atom positions.

Md. Mijanuddin et al. / Journal of Molecular Structure 693 (2004) 161–165
163
Table 2
˚
Selected bond lengths (A) and angles (8)
hydrogen bridges with aliphatic hydrogen atoms of two
adjacent molecules, thus forming a polymeric chain (C1–
˚
H1· · ·O7: 3.366(2) A and 1678) along the a-axis (Fig. 2).
N1–C1
1.284(2)/1.307
1.404(2)/1.345
1.454(2)/1.462
1.350(2)/1.367
121.40(1)/123.0
119.3/123.5
N1–N1_a
C1–C2
Although, C1–H1· · ·O7 distance seems to be large
compared to the normal intermolecular hydrogen bonding
systems, recent experiments have revealed that the C· · ·O
C7–O7
N1–C1–C2
N1–C1–H1
C1–N1–N1_a
C2–C1–H1
C3–C2–C1
C7–C2–C1
distance in C–H· · ·O hydrogen bond can be as large as
˚
4.0 A [18]. There is no visible p–p interaction between
such a chain and its two immediate neighbors, as the
113.51(1)/118.7
119.3/113.6
120.05(11)/116.3
121.59(11)/125.6
˚
distance between neighboring benzene ring is ,6.5 A.
AM1 optimization of the structure of salicylaldazine
molecule assigns a C2h symmetry to the molecule. This is
supported by the crystal structure analysis. This leads to the
possibility of the molecular system being vulnerable to a
single and/or DPT reaction in the ground state and/or excited
state. To examine theoretically, the possibilities of these
processes, we have simulated the potential energy curves
reasonable agreement with the X-ray structural data. The
deviation between the two sets is assigned to the crystal field
stabilization and intra and intermolecular hydrogen bonding
effects. Fig. 1 shows the ORTEP view with atom numbering
of the salicylaldazine, which was drawn using 25%
probability ellipsoids.
(
PEC) for the prototropic processes in both ground state (S0)
and the lowest excited singlet state (S ). For the intramole-
1
The basic features of the molecular structure do not differ
from those of the expected structure. The two benzene rings
are coplanar and are in trans configuration with C1–N1 and
C1_a–N1_a bonds relative to N1–N1_a, which may be due
to stabilization via intramolecular O–H· · ·N hydrogen
bonds between N1 and H7–O7 and also N1_a and H7_a–
O7_a· · ·O7–H7 acts as a donor in the three-centre O7–
H7· · ·N1 interaction which is intramolecular in nature, and
coplanar with the adjacent benzene ring (C1–N1–N1_a–
C1_a: 179.98(12)8, N1_a–N1–C1–C2: 179.88(11)8, N1–
C1–C2–C3: 179.54(12)8, N1–C1–C2–C7: 1.32(19)8). In a
second hydrogen bond, O7 acts as an acceptor in a three-
centre C1–H1· · ·O7 interaction, which is intermolecular in
nature, connecting the molecule to a symmetry-related one
cular single proton transfer (SPT) reaction, the N1–H7
ðR Þ distance has been considered as the reaction
1–7
coordinate. For the intramolecular DPT both N1–H7
ðR Þ and N1_a–H7_a distances have been treated as
1–7
reaction coordinates. However, considering the symmetric
structure of the molecule, variation of both the distances has
been kept same in each stage. For the generation of the PEC
for the IPT process, we have optimized the geometry with
various preset values of the reaction coordinate. Fig. 3
depicts the simulated PECs for the intramolecular single and
DPT processes of the isolated molecule in the S and S
0 1
states. The figure clearly reveals that the tautomer
formation through intramolecular SPT in the ground state
2
1
(
symmetry code: 21=2 þ x; 1=2 2 y; 21=2 þ zÞ:
leads to endothermicity (DHcalc: ¼ 10:4 kcal mol ).
The structure determination reveals that the 2D crystal
structure consists of individual chains. In each chain, two
phenolic oxygen atoms of each molecule form very weak
However, the reaction becomes exothermic in the S
1
2
1
state (DHcalc: ¼ 29:2 kcal mol ). Thus, the SPT reaction
is thermodynamically unfavorable in the ground state but it is
Fig. 1. ORTEP view with atom numbering of the salicylaldazine, drawn using 25% probability ellipsoids.

164
Md. Mijanuddin et al. / Journal of Molecular Structure 693 (2004) 161–165
Fig. 2. Formation of chains through weak intermolecular C–H· · ·O hydrogen bridges.
favored in the S state. Considering the kinetic aspect of the
1
same reaction the calculation reveals that the activation
2
1
energy for the process is quite high (21.9 kcal mol ) which
is unattainable under the normal situations. This high
activation barrier imposes a kinetic restriction on the
occurrence of the process in the ground state. This barrier
is, however, reduced appreciably in the lowest excited singlet
2
1
state. The calculated activation barrier is 11.2 kcal mol
which is nearly half the barrier experienced in the ground
state. Thus, both the thermodynamic as well as the kinetic
factors suggest that the intramolecular SPT is feasible in the
S1 state although it is improbable in the ground state.
A similar treatment for the intramolecular DPT reaction,
as represented in the same figure, indicates that the DPT
process is endothermic in both the S and S states amounting
0
1
2
1
to DHcalc: ¼ 29:6 and 13.6 kcal mol , respectively.
Although, the endothermicity is reduced remarkably in the
excited state, thermodynamics do not favor the IDPT process
Fig. 3. Potential energy curves for the proton transfer process of
salicylaldazine in S and S states. SPT and DPT represent single and
double proton transfer, respectively.
0
1

Md. Mijanuddin et al. / Journal of Molecular Structure 693 (2004) 161–165
165
in either of the two states. The activation energies for the
2
IDPT process are calculated to be 55.6 and 38.4 kcal mol
[3] J.Y. Ying, C.P. Mehnert, M.S. Wong, Angew. Chem. Int. Ed. 38
(1999) 56–77.
1
[
4] G.A. Jeffrey, in: an Introduction to Hydrogen Bonding, Oxford
University Press, New York, 1997.
in S0 and S1 states, respectively. Both of the barriers
corresponding to the two electronic states are too high to
allow a kinetic process to take place. It is thus apparent that
the DPT process is not feasible in either of the S and S states
[
5] G.A. Jeffrey, W. Saenger, in: Hydrogen Bonding in Biological
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1
[
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because of both thermodynamic and kinetic factors.
The simulated PEC, thus, suggest that salicylaldazine is
susceptible to excited state intramolecular SPT but in spite of
being symmetric the corresponding DPT is unfavorable both
from thermodynamic as well as kinetic points of view.
[
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Thiel, Chem. Phys. 163 (1992) 43–53.
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