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Synthesis, characterization, crystal structure and theoretical studies on 4-bromo-2-[(E)-6-methyl-2-pyridyliminomethyl]phenol
⁎Corresponding author. Tel.: +98 171 2245882. alidkhalaji@yahoo.com (Aliakbar Dehno Khalaji) ad.khalaji@gu.ac.ir (Aliakbar Dehno Khalaji)
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Received: ,
Accepted: ,
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.
Peer review under responsibility of King Saud University.
Abstract
The Schiff base compound, 4-bromo-2-[(E)-6-methyl-2-pyridyliminomethyl]phenol, has been synthesized by a reaction of 5-bromosalicylaldehyde with 6-methyl-2-aminopyridine and characterized by elemental analyses, 1H-NMR spectroscopy and X-ray single-crystal diffraction. The title compound crystallizes in monoclinic system, space group P21/c, with a = 13.2721(12) Å, b = 4.5481(4) Ǻ, c = 19.972 (2) Ǻ, β = 96.653 (3)°, V = 1197.4 (2) Ǻ3 and Z = 4. The molecular geometry obtained from X-ray crystallography has been compared with that of the title compound in the ground state calculated by using the density function method (B3LYP) with 6/31G basis set. Calculated results show that DFT can well reproduce the structure of the title compound. The DFT results show that the molecule exists only in enol form. The molecular conformation of the title compound is stabilized by an intramolecular O1–H1·N1 hydrogen bond.
Keywords
Schiff base
Single-crystal
Spectroscopy
DFT
Hydrogen bond
1 Introduction
The ortho-hydroxy Schiff bases, which show tautomerism by an intramolecular proton transfer from an oxygen atom to the neighboring imine nitrogen atom, are important compounds (Albayrak et al., 2010, 2011; Tanak et al., 2010; Hamaker et al., 2010; Tunc et al., 2009; Petek et al., 2010; Unver et al., 2009), because they can exist in three different structures as enol, keto or zwitterionic forms in the solid state (Hadjoudis et al., 2004; Unver et al., 2005; Yildiz et al., 2005) and show thermochromism and photochromism (Hadjoudis et al., 2004), antimicrobial activity (Unver et al., 2005; Yildiz et al., 2005) and usage as ligands in the field of coordination chemistry (Ran et al., 2008; Senol et al., 2011; Khalaji et al., 2011a; Mandal et al., 2009; Song et al., 2008). In general, Schiff bases display two possible tautomeric forms, the phenol-imine (enol) (Albayrak et al., 2010; Tanak et al., 2010) and keto-amine (keto) forms (Albayrak et al., 2011).
As an additional contribution to synthesis and characterization of Schiff base compounds and in the course of our ongoing studies on these kinds of meterials (Khalaji et al., 2010, 2011a, 2011b, 2011c), we describe here the synthesis, characterization, crystal structure and quantum chemical calculational studies of the Schiff base compound 4-bromo-2-[(E)-6-methyl-2-pyridyliminomethyl]phenol (Scheme 1).![Chemical structure of the title compound 4-bromo-2-[(E)-6-methyl-2-pyridyliminomethyl]phenol.](/content/184/2017/10/2_suppl/img/10.1016_j.arabjc.2013.07.007-fig1.png)
Chemical structure of the title compound 4-bromo-2-[(E)-6-methyl-2-pyridyliminomethyl]phenol.
2 Experimental
2.1 Physical techniques and materials
All reagents and solvents for synthesis and spectroscopic studies were commercially available and used as received without further purification. Elemental analyses were carried out using a Heraeus CHN-O-Rapid analyzer. 1H-NMR spectra were measured with a BRUKER DRX-500 AVANCE spectrometer at 500 MHz and all chemical shifts are reported in δ units downfield from TMS.
2.2 Synthesis of 4-bromo-2-[(E)-6-methyl-2-pyridyliminomethyl]phenol
6-Methyl-2-aminopyridin (1 mmol) and 5-bromo-salicylaldehyde (1 mmol) were dissolved in methanol (10 ml) at 323 K. The mixture was stirred for 2 h to give a clear yellow solution. After keeping the solution in air for several days by slow evaporation of the solvent, yellow blocks of the title compound were formed, with 85% yield. The crystals were isolated, washed several times with methanol and dried in a vacuum desiccator containing anhydrous CaCl2. Anal. Calc. for C13H11BrN2O: C, 53.63; H, 3.81; N, 9.62%. Found: C, 53.78; H, 3.85; N, 9.51%. 1H-NMR (CDCl3, δ(ppm)): 2.58 (s, 3H), 6.93 (d, 1H), 7.11 (t, 2H), 7.46 (dd, 1H), 7.65 (m, 2H), 9.39 (s, 1H), 13.58 (s, 1H).
2.3 X-ray single crystal analysis
Crystallographic measurements were done with Rigaku RAXIS-RAPID II diffractometer, with graphite monochromataed Mo Kα radiation (λ = 0.71075 Å) at 180 K. A numerical absorption correction was applied for the data (Higashi, 1999). The crystal structure of 4-bromo-2-[(E)-6-methyl-2-pyridyliminomethyl]phenol was solved by direct methods using program SHELXS97 (Sheldrick, 2008) and refined by full-matrix least-squares technique based on F2. The C-bound H atoms were placed in geometrically idealized positions (C—H = 0.95 or 0.98 Å) and allowed to ride on their parent atoms, with Uiso(H) = 1.2Ueq(C) or 1.5Ueq(methyl C). The O-bound H atom was located in a difference Fourier map and the positional parameters were refined, with Uiso(H) = 1.2Ueq(O). Crystallographic data and details of the data collection and structure refinement are listed in Table 1. The molecular structure plots were prepared by ORTEP-3 (Farrugia, 1997).
Empirical formula
C13H11BrN2O
Formula weight
291.15
Crystal system
Monoclinic
Space group
P21/c
a (Å)
13.2721 (12)
b (Å)
4.5481 (4)
c (Å)
19.972 (2)
β (deg)
96.653 (3)
V (Å3)
1197.4 (2)
Z
4
μ (mm−1)
3.43
Tmin
0.473
Tmax
0.735
Measured reflections
20716
Independent reflections
3463
Reflection with I > 2σ(I)
2131
Parameters
158
Rint
0.057
R[F2 > 2σ(F2)]
0.059
wR(F2)
0.202
S
1.122
Δρmax (eÅ−3)
0.73
Δρmin (eÅ−3)
−1.55
2.4 Theoretical methods
DFT calculations with a hybrid functional B3LYP at 6-31G basis set using the Berney method (Schlegel, 1982; Peng et al., 2009) were performed with the Gaussian 98 R-A.9 software package (Frisch et al., 1998).
3 Results and discussion
3.1 Synthesis and spectroscopy
The title compound 4-bromo-2-[(E)-6-methyl-2-pyridyliminomethyl]phenol was prepared in yield of 85% in a methanolic solution. It is air-stable in the solid state for about 8 months. The stability of the dissolved compound is much shorter than in the solid state and depends on the nature of the solvent. The title compound is very slightly soluble in common organic solvents such as acetonitrile and methanol but completely soluble in chloroform and dichloromethane.
The 1H-NMR spectrum of 4-bromo-2-[(E)-6-methyl-2-pyridyliminomethyl]phenol was recorded using CDCl3 as the solvent and data are summarized in the experimental section and shown in Fig. 1. The hydrogen of azomethine group (Hb) was shown at δ = 9.39 ppm as a singlet. Methyl protons appear as singlet at δ = 2.58 ppm. Aromatic ring protons are shown in the range δ = 6.92–7.68 ppm. The hydrogen of iminic protons and hydroxy group were shown at δ = 9.38 ppm and δ = 13.58 ppm, respectively, as a singlet signals.![1H NMR spectra of 4-bromo-2-[(E)-6-methyl-2-pyridyliminomethyl]phenol.](/content/184/2017/10/2_suppl/img/10.1016_j.arabjc.2013.07.007-fig2.png)
1H NMR spectra of 4-bromo-2-[(E)-6-methyl-2-pyridyliminomethyl]phenol.
3.2 Crystal structure
An ORTEP-3 view of 4-bromo-2-[(E)-6-methyl-2-pyridyliminomethyl]phenol with the atom numbering scheme is given in Fig. 2. Bond lengths and angles are given in Tables 2–4, along with the calculated bond parameters. All bond lengths and angles in 4-bromo-2-[(E)-6-methyl-2-pyridyliminomethyl]phenol are in the normal range (Aygun et al., 2004).![An ORTEP view 4-bromo-2-[(E)-6-methyl-2-pyridyliminomethyl]phenol with O1–H1···N1 hydrogen bond (dashed line).](/content/184/2017/10/2_suppl/img/10.1016_j.arabjc.2013.07.007-fig3.png)
An ORTEP view 4-bromo-2-[(E)-6-methyl-2-pyridyliminomethyl]phenol with O1–H1···N1 hydrogen bond (dashed line).
X-ray
DFT/B3LYP (enol)
DFT/B3LYP (keto)
Br1-C4
1.898 (3)
1.950
1.948
O1–C1
1.345 (5)
1.358
1.267
O1–H1
1.00 (5)
1.020
-
N1–C7
1.286 (4)
1.308
1.361
N1–C8
1.420 (4)
1.414
1.417
N2–C8
1.334 (4)
1.350
1.339
N2–C12
1.345 (4)
1.352
1.354
C3–C4
1.374 (4)
1.381
1.358
C12–C13
1.490 (6)
1.506
1.504
C1–C6
1.392 (5)
1.405
1.458
C1–C2
1.415 (5)
1.427
1.478
C2–C3
1.390 (5)
1.413
1.445
C8–C9
1.381 (5)
1.402
1.402
C9–C10
1.371 (5)
1.395
1.393
C2–C7
1.446(4)
1.444
1.386
C10–C11
1.372 (6)
1.399
1.399
C11–C12
1.401 (6)
1.405
1.401
C5–C6
1.379 (5)
1.390
1.365
C4–C5
1.392 (4)
1.401
1.430
X-ray
DFT/B3LYP (enol)
DFT/B3LYP (keto)
C1–O1–H1
106 (2)
108.05
–
C7–N1–C8
120.2 (3)
121.12
129.4
C8–N2–C12
118 (3)
119.5
118.9
O1–C1–C6
119 (3)
119.1
120.8
O1–C1–C2
121.4 (3)
121.06
122.8
C6–C1–C2
119.6 (3)
119.7
116.2
C3–C2–C1
118.7 (3)
118.9
118.8
C3–C2–C7
119.8 (3)
120.1
116.4
C1–C2–C7
121.4 (3)
120.8
124.4
C4–C3–C2
120.7 (3)
119.9
120.7
C3–C4–C5
120.8 (3)
121.2
121.6
C3–C4–Br1
119.8 (2)
119.5
120.3
C5–C4–Br1
119.4 (2)
119.2
117.9
N2–C8–C9
123.3 (3)
122.5
123.4
N2–C8–N1
119.1 (3)
119.2
117.2
C9–C8–N1
117.5 (3)
118.2
119.2
C10–C9–C8
118.8 (3)
117.3
117.5
C9–C10–C11
119.1 (3)
119.9
119.4
C10–C11–C12
119.3 (3)
119
119.2
N2–C12–C11
121.5 (3)
122
121.2
N2–C12–C13
117.5 (3)
115.9
116.3
C11–C12–C13
121.0 (3)
121.9
122.3
C6–C5–C4
119.3 (3)
118.3
119.8
C5–C6–C1
120.8(3)
120.7
122.3
N1–C7–C2
121.6 (3)
179.5
121.3
X-ray
DFT/B3LYP (enol)
DFT/B3LYP (keto)
O1–C1–C2–C3
−179.0 (3)
−179.9
−179.4
C6–C1–C2–C3
0.4 (4)
0.02
5.6
O1–C1–C2–C7
1.5 (5)
0.02
3.4
C6–C1–C2–C7
−179.0 (3)
−179.9
−179.4
C1–C2–C3–C4
0.1 (4)
−0.03
−4.3
C7–C2–C3–C4
179.5 (3)
179.9
179.6
C2–C3–C4–C5
−0.8 (4)
0.01
0.7
C2–C3–C4–Br1
178.8 (2)
179.9
179.2
C3–C4–C5–C6
1.0 (5)
0.01
1.4
Br1–C4–C5–C6
−178.6 (2)
−179.9
−178.6
C4–C5–C6–C1
−0.5 (5)
0.02
0.2
O1–C1–C6–C5
179.3 (3)
−179.9
173.4
C2–C1–C6–C5
−0.2(5)
0.01
−3.7
C8–N1–C7–C2
179.9 (3)
−33
26.7
C3–C2–C7–N1
177.5 (3)
16.9
−170
C1–C2–C7–N1
−3.0 (4)
−163
14.2
C12–N2–C8–C9
0.9 (5)
0.0
2.18
C12–N2–C8–N1
−179.9 (3)
179.9
177.6
C7–N1–C8–N2
−10.9 (4)
16
28.4
C7–N1–C8–C9
168.4 (3)
−164
−155.9
N2–C8–C9–C10
−1.1 (5)
−0.01
−0.8
N1–C8–C9–C10
179.7 (3)
−179.9
−176.2
C8–C9–C10–C11
0.5 (5)
0.01
0.5
C9–C10–C11–C12
0.2 (6)
−0.01
0.4
C8–N2–C12–C11
−0.1 (5)
0.01
−2.2
C8–N2–C12–C13
178.6 (3)
−179.9
177.3
C10–C11–C12–N2
−0.4 (5)
−0.01
0.9
C10–C11–C12–C13
−179.1 (4)
179.9
−178.5
The N1-C7 distance of 1.286(4) Ǻ is within the range of a double C⚌N bond, while the N1-C8 distance of 1.420(4) Ǻ is comparable to single C–N bond distances in other Schiff-base compounds (Karakas et al., 2005, 2008). The bond angles N1-C7-C2 and C7-N1-C8 are 121.6(3) and 120.2(3)°, respectively, and they are consistent with the sp2 hybrid character for C7 and N1 atoms (Karakas et al., 2005, 2008). The benzene C1-C6 plane and the nimine C7⚌N2 group are approximately planar, as indicated by the torsion angles of C3-C2-C7-N1, C1-C2-C7-N1 and C8-N1-C7-C2 being 177.5(3), −3.0(4) and 179.9(3)°, respectively, and the dihedral angle of 3.0(3)° between the C1 and C6 and C7/N1/C8 planes. The planarity is supported by an intramolecular O1–H1···N1 hydrogen bond between hydroxy oxygen atom and imine nitrogen atom (Fig. 2; O1–H1 = 1.01, H1–N1 = 1.68(4), O1...N1 = 2.600(4) Ǻ and <O1–H1...N1 = 150(4)°]. However, the benzene and pyridine rings are slightly twisted with a dihedral angle of 14.13(16)° between them.
3.3 Theoretical studies
B3LYP/6-31G calculation was performed on the title compound. The B3LYP/6-31G calculation shows that the N–H (keto) form of the title compound is less stable than its O–H (enol) form in the gas phase. Calculated geometrical parameters of enol (Fig. 3) and keto (Fig. 4) forms of the title compound are listed in Tables 2–4 along with the experimental data. As seen from Table 2, most of the optimized bond distances are slightly longer than the experimental values. We noted that the experimental results belong to the solid phase and theoretical calculations belong to the gas phase. The corresponding C1–O1 and C7–N1 distances of enol formed at the optimized geometry are 1.358 and 1.308 Ǻ while these distances for keto form are 1.267 and 1.361 Ǻ, respectively. From X-ray single-crystal data, C–O distance of 1.345(5) Ǻ and C–N distance of 1.286(4) Ǻ indicate single and double bond characters, respectively. The results obtained from the DFT calculation for enol form are in reasonable agreement with the experimental as seen.
The B3LYP/6-31G optimized enol form of the title compound.
The B3LYP/6-31G optimized keto form of the title compound.
Acknowledgments
We acknowledge Golestan University (GU) for partial support of this work, Okayama University (OU) and the project Praemium Academiae of the Academy of Sciences (ASCR).
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