Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 145 (2015) 563–574

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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa

Syntheses, crystal structure, Hirshfeld surfaces, fluorescence properties, and DFT analysis of benzoic acid hydrazone Schiff bases Mohammad Sayed Alam a,b, Dong-Ung Lee a,⇑ a b

Division of Bioscience, Dongguk University, Gyeongju 780-714, Republic of Korea Department of Chemistry, Jagannath University, Dhaka 1100, Bangladesh

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 Benzoic acid hydrazone Schiff bases

have been synthesized and characterized by physical, spectral and analytical data.  Intermolecular interactions in the crystal packing have been studied using Hirshfeld surface analyses.  Vibrational frequencies, molecular electrostatic potential map, frontier molecular orbitals have been studied.  The fluorescence properties with varying solvent polarities were explained using the results of DFT analyses.

a r t i c l e

i n f o

Article history: Received 3 February 2015 Received in revised form 3 March 2015 Accepted 5 March 2015 Available online 13 March 2015 Keywords: Hydrazone Schiff base Crystal structure DFT-B3LYP Fluorescence

⇑ Corresponding author. Fax: +82 54 742 9833. E-mail address: [email protected] (D.-U. Lee). http://dx.doi.org/10.1016/j.saa.2015.03.071 1386-1425/Ó 2015 Published by Elsevier B.V.

LUMO (-1.744 eV)

HOMO (-6.274 eV)

LUMO (-1.697 eV)

HOMO (-6.187 eV)

a b s t r a c t Two hydrazone Schiff base analogues, namely, (E)-N0 -(4-hydroxy-3-methoxybenzylidene)benzohydrazide (3a) and (E)-N0 -(4-methoxybenzylidene)benzohydrazide (3b), were synthesized using a mild, efficient method and characterized by 1H NMR, mass spectrometry, elemental analysis, and single-crystal X-ray diffraction. X-ray analysis of a single crystal of 3a revealed a tetragonal, space group I4(1)/a structure, with an E-configuration around the azomethine (AC8@N2A) double bond. In this structure, the ANHA and AOH groups act as proton donors and the >C@O and AN@ groups as proton acceptors, and these facilitate hydrogen bond formation in the crystal state. Plausible intermolecular interactions were studied using 3D Hirshfeld surfaces and related 2D fingerprint plots. The optimized geometry, vibrational frequencies, Mulliken charge distribution, molecular electrostatic potential (MEP) maps, frontier molecular orbitals (FMOs), and associated energies of the ground state and the first single excited state were calculated using density functional theory (DFT) and time-dependant DFT calculations using the B3LYP/6311G method. Vibrational frequencies calculated in the gaseous phase compared with experimental values measured in the solid state and showed good agreement with each other. The chemical reactivities of 3a and 3b were predicted by mapping MEP surface over optimized geometries and comparing these with MEP map generated over crystal structures. Mulliken charge distribution analysis and MEP map of 3a and 3b revealed that N(1), O(1), O(2) and O(3) atoms could act as electron donors and coordinate with metals and that these represented the most suitable sites for electrophilic attack. In fluorescence spectra, the absorption and emission spectra of 3a and 3b were similar in different polar solvents with few exceptions. In addition, both compounds exhibited dual emission spectra in acetone due to keto-enol tautomerism induced by photoexcitation. Ó 2015 Published by Elsevier B.V.

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Procedure for the preparation of benzoic acid hydrazide Schiff base analogues (3)

Introduction Schiff bases were named after Hugo Schiff in the 19th century and are synthesized by condensation between a primary amine and an aldehyde or ketone to form an azomethine or imine (ACH@NA) group [1]. Schiff bases have attracted considerable attention due to their wide-ranging applications, such as, in liquid crystals [2], organic dyes [3,4], catalysts [5], and as intermediates for many bioactive molecules [6,7]. The hydrazones are a type of Schiff base, and are characterized by the presence of an open chain ARR1NN@CR2R3A group, and are used as major building blocks for the syntheses of a variety of useful bioactive compounds. Hydrazones have been reported to possess many biological activities, including, anticancer [8], antibacterial [9,10], anti-tuberculosis [11], anti-convulsant [12], anti-oxidant [13], and diuretic properties [14,15], and have been used as herbicides, insecticides, nematocides, rodenticides, plant growth regulators, and as sterilants for houseflies [16,17]. Moreover, metal complexes of hydrazide–hydrazones are important catalysts, especially of many important enzymatic reactions [18–21] and also showed wide range of bioactivities and applications [22–25]. Accordingly, hydrazone Schiff bases and their complexes have attracted considerable research attention. In a previous study [26], we reported the synthesis, antioxidant activity and fluorescence properties of Eu(III) complexes of naphthyledine–hydrazone Schiff base and elucidated their coordination properties using density functional theory (DFT). Here, we describe the syntheses, crystal structures, Hirshfeld surface analyses, and fluorescence properties of two hydrazone Schiff bases. Frontier molecular orbital (FMO) and Mulliken charge distribution (MCD) analysis were carried out to elucidate information regarding charge transfer within HOMO–LUMO of the molecule and its coordination properties. In addition, the molecular electrostatic potential (MEP) surface of 3a and 3b was also predicted using optimized geometry and crystal structure to elucidate the reactivity of hydrazone Schiff base analogue.

Benzoic acid hydrazide Schiff base analogues (3a and 3b) were prepared as previously described [26]. Briefly, benzohydrazide (2) was prepared by refluxing a mixture of methyl benzoate (1.36 g, 10 mmol) and hydrazine hydrate (1.25 g, 25 mmol) in ethanol (30 mL) for 3 h. Precipitated benzohydrazide was filtered and recrystallized from an ethanol–water mixture. Then, benzohydrazide (2, 3 mmol) in 30 mL ethanol was added drop-wise into 20 mL of 4-hydroxy-3-methoxy- (3 mmol) or 4-methoxybenzaldehyde (3 mmol) in ethanol and refluxed with stirring for 2–2.5 h. Reaction progress was monitored by TLC. After cooling to ambient temperature, reaction mixtures were filtered to give solid crude products, which were crystallized from ethanol to obtain pure compounds. (E)-N0 -(4-Hydroxy-3-methoxybenzylidene)benzohydrazide (3a): Yield: 88%, m.p. 167–68 °C (white crystal). IR (cm1): 3430 (OH), 3231 (NH), 2919 (CH), 1700 (C@O). 1H NMR (DMSOd6, ppm): d 3.83 (s, 3H, AOCH3), 6.83–7.11 (m, 1H, Ar-H), 7.32–7.55 (m, 5H, Ar-H), 7.88 (d, 2H, J = 3.6 Hz, Ar-H), 8.34 (s, 1H, CH), 11.66 (s, 1H, NH). EI–MS m/z (%): 270 (M+, 32), 105 (100); Anal. calcd. for C15H14N2O3: C, 66.66; H, 5.22; N, 10.36. Found: C, 66.73; H, 5.29; N, 10.41. (E)-N0 -(4-Methoxybenzylidene)benzohydrazide (3b): Yield 89%, m.p. 129–30 °C (white powder). IR (cm1): 3189 (NH), 2904 (CH), 1647(C@O). 1H NMR (DMSOd6, ppm): d 3.82 (s, 3H, AOCH3), 7.52–7.63 (m, 5H, Ar-H), 6.8 (d, 2H, J = 7.3 Hz, Ar-H), 7.6 (d, 2H, J = 7.3 Hz, Ar-H), 8.1 (s, 1H, CH), 10.90 (s, 1H, NH). EI–MS m/z (%): 254 (M+, 17), 105 (100); Anal. calcd. for C15H14N2O2: C, 70.85; H, 5.55; N, 11.02. Found: C, 70.93; H, 5.62; N, 11.10. X-ray crystallography White rod-shaped crystals of 3a were grown by slow evaporation from ethanol and a single crystal of size 0.27 mm  0.17 mm  0.06 mm was chosen for the X-ray diffraction study. Data collection was carried out using a Bruker SMART CCD detector and k (MoKa) = 0.71073 Å [27]. A total of 16855 reflections were collected, of which 2680 (22 6 h 6 21, 22 6 k 6 22, 17 6 l 6 19) were treated as observed. The coordinates of non-hydrogen atoms were determined by direct methods using SHELXTL NT Version 6.12 [28]. Full-matrix least-squares refinement using SHELXTL with isotropic displacement parameters for all non-hydrogen atoms converged the residual to R1 = 0.1536. Subsequent refinements [29] were carried out using anisotropic thermal parameters for non-hydrogen atoms. Hydrogen atoms were fixed at chemically acceptable positions and were allowed to ride on parent atoms at NAH and CAH distances of 0.8800–0.9800 Å. The final refinement converged to R = 0.1071, wR = 0.3035, w = 1/[r2(F0)2 + (0.2000P)2 + 0.00P], where P = (max(F20 + 2F2c )/3), r = 1.154, (D/r)max < 0.001, (Dq)max = 0.3483 and (Dq)min = 0.390 e Å3.

Experimental details General Melting points were determined using a Stuart SMP3 apparatus, and were uncorrected. FT-IR spectra were obtained using a Bruker Tensor 37 spectrometer using KBr discs. NMR spectra were recorded using a Bruker 400 MHz spectrometer using TMS as the internal standard. Mass spectra were acquired using a Jeol JMS700 high-resolution mass spectrometer at the Korea Basic Science Center (Daegu, South Korea). Elemental analyses (C, H, N) were performed on a Perkin Elmer 2400 II CHN elemental analyzer. Electronic absorption spectra were obtained using a Varian Cary 4000 spectrophotometer and fluorescence spectra using a Varian Cary Eclipse Fluorescence spectrophotometer. O

COOCH3 NH2-NH2.H2O

N H

EtOH, 1h, 80 oC

NH2

OHC

O

R

N H

EtOH, 1.5 h, 80 oC

OCH3 OH

O N H

N

3a

R 3

2

1

N

O N H

N

OCH3

3b

Scheme 1. The synthesis of novel benzoic acid hydrazone Schiff base analogues.

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Computational methods All density functional theory (DFT) and time-dependant DFT calculations were conducted with a hybrid functional B3LYP (Becke’s three parameter nonlocal exchange function with the Lee–Yang–Parr correlation function) [30,31] at 6-311G basis set using the GAMESS interface in ChemBio3D ultra ver. 14.0 (PerkinElmer, MA, USA). No imaginary frequencies were obtained during vibrational frequency calculations, indicating that all structures were stable. After optimization, Mulliken charge, molecular electrostatic map, and properties of frontier molecular orbitals of 3a and 3b were analyzed using results calculated at the B3LYP/6311G level. Hirshfeld surfaces and related 2D fingerprint plots, and the molecular electrostatic map surface over the crystal structure of 3a were calculated using Crystal Explorer 3.1 [32] and Tonto [33].

Results and discussion Synthesis and crystal structure The desired benzoic acid hydrazone Schiff base analogues (3a and 3b) were prepared using straightforward, convenient two step reactions as presented in Scheme 1, and their structures were elucidated by FT-IR, 1H NMR, mass spectroscopy, and elemental analyses. The FT-IR spectra of compounds 3a and 3b showed absorption bands in the 3430 and 3189–3231 cm1 regions resulting from AOH and ANHA stretch, respectively. The absorption band of >C@O stretch appeared at 1647–1700 cm1. The 1H NMR spectra of 3a and 3b showed singlet at 8.34 and 8.10 ppm corresponding to the ACH@NA proton, respectively and another singlet at 10.90 and 11.66 ppm corresponding to the @NANHA proton, respectively. Aromatic protons and protons of other functional groups produced peaks in accord with substitution patterns. The EI–MS spectra of 3a and 3b showed molecular ion peaks of intensity 32% and 17%, respectively. Compound 3a was crystallized from ethanol as single white crystals using the slow evaporation technique and analyzed by Xray diffraction method. An ORTEP drawing of the molecular structure of 3a at a 50% probability, with its numbering scheme, is shown in Fig. 1. X-ray diffraction data and refinements are presented in Table 1, and selected bond lengths and angles are listed in Tables 2 and 3. Detailed atomic coordinates, thermal parameters, and

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Table 1 Crystal structure and refinement data for (E)-N0 -(4-hydroxy-3-methoxybenzylidene) benzohydrazide (3a). Empirical formula Formula weight Temperature Wavelength Crystal system Space group Unit cell dimensions Volume Z Density (calculated) Absorption coefficient F(0 0 0) Crystal size Theta range for data collection Index ranges Reflections collected Independent reflections Completeness to theta = 25.99° Absorption correction Max. and min. transmission Refinement method Data/restraints/parameters Goodness-of-fit on F2 Final R indices [I > 2r (I)] R indices (all data) Largest diff. peak and hole CCDC deposit number

C15H14N2O3 270.28 200(2) K 0.71073 Å Tetragonal I4(1)/a a = 18.6378(4) Å b = 18.6378(4) Å c = 15.6727(8) Å a = b = c = 90° 5444.2(3) Å3 16 1.319 Mg/m3 0.093 mm1 2272 0.27  0.17  0.06 mm3 1.70–25.99° 22 6 h 6 21, 22 6 k 6 22, 17 6 l 6 19 16,855 2680 [R(int) = 0.0597] 100.0% None Full-matrix least-squares on F2 Full-matrix least-squares on F2 2680/0/183 1.154 R1 = 0.1071, wR2 = 0.3035 R1 = 0.1536, wR2 = 0.3483 1.236 and 0.390 e Å3 CCDC 1020312

torsion angles are provided as Supporting Information. Compound 3a crystallized in the tetragonal space group I4(1)/a and was found to adopt an E-configuration about its AC(8)@N(2)A double bond. The two phenyl rings of 3a were found to be linked by a open chain carbonyl-hydrazone (AC@NANHACOA) system, in which C8, N2, N1, C7, and O1 atoms were almost in the same plane with a dihedral angle of 9.9(6)° between O(1)–C(7)–N(1)–N(2) and of 6.5(4)° between C(8)–N(2)–N(1)–C(7). In addition, the 3-methoxy-4-hydroxy-benzylidenephenyl ring of 3a was almost coplanar with the plane formed by AC(8)@N(2)A with a dihedral angle of 15.5(7)°. On the other hand, the benzoyl phenyl ring (C1–C6) of 3a was significantly rotated out of the plane formed by the remainder of the molecule at an angle of 39.2(7)°. The torsion angle values of N(2)–C(8)–C(9)–C(10) and C(2)–C(1)–C(7)–N(1) were 168.8(4)° and 138.4(5)°, respectively.

Fig. 1. ORTEP drawing of (E)-N0 -(4-hydroxy-3-methoxybenzylidene) benzohydrazide (3a) and its numbering scheme. Thermal ellipsoids are drawn at the 50% probability level at 200 K.

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Table 2 Selected interatomic distances (Å) and valence angles (°) as determined by X-ray crystallography and theoretical calculations for 3a and 3b using B3LYP/6-311G level of theory. Bonding atoms

Bond lengths (Å)

Bonding atoms

3a

C(1)–C(6) C(1)–C(2) C(1)–C(7) C(2)–C(3) C(3)–C(4) C(4)–C(5) C(5)–C(6) C(7)–O(1) C(7)–N(1) N(1)–N(2) N(1)–H(1A) N(2)–C(8) C(8)–C(9) C(9)–C(10) C(9)–C(14) C(10)–C(11) C(11)–C(12) C(12)–O(2) C(12)–C(13) C(13)–O(3) C(13)–C(14) O(3)–C(15) O(2)–C(15)

3b

3a

B3LYP

Exp.

B3LYP

B3LYP

Exp.

B3LYP

1.385 1.423 1.522 1.395 1.423 1.388 1.399 1.245 1.364 1.417 0.97 1.297 1.472 1.395 1.396 1.395 1.395 1.355 1.383 1.355 1.389 1.402 –

1.336(7) 1.424(8) 1.489(6) 1.390(9) 1.441(10) 1.344(10) 1.406(9) 1.234(5) 1.335(6) 1.394(5) 0.8800 1.281(6) 1.461(6) 1.389(7) 1.402(6) 1.382(7) 1.381(7) 1.356(5) 1.392(7) 1.366(5) 1.379(6) 1.408(7) –

1.396 1.405 1.500 1.397 1.391 1.393 1.400 1.230 1.390 1.365 0.98 1.293 1.476 1.397 1.395 1.399 1.426 1.367 1.399 – 1.395 – 1.423

120.566 124.298 119.126 119.241 119.616 120.003 120.308 120.081 125.973 121.474 116.602 119.325 118.127 119.696 120.455 121.441 119.233 120.654 119.830 120.452 121.987 120.103 123.935 115.802 119.262 118.236 116.009

117.4(5) 124.5(5) 118.1(4) 119.7(6) 118.3(6) 120.1(6) 122.6(6) 121.7(6) 122.6(4) 121.4(4) 115.9(4) 119.8(4) 114.4(4) 122.1(4) 119.2(4) 120.0(4) 120.7(4) 121.0(5) 119.8(5) 119.2(4) 121.2(4) 119.6(4) 124.7(4) 114.4(4) 121.0(4) 119.4(4) 118.6(4)

118.944 124.868 117.402 120.979 119.906 119.841 120.400 120.608 122.624 118.811 117.318 119.138 116.199 121.345 118.129 120.717 119.141 120.104 119.894 124.240 114.621 119.626 – – 120.804 119.457 116.869

Table 3 Selected dihedral angles (°) as determined by X-ray crystallography and theoretical calculations for 3a and 3b at B3LYP/6-311G level of theory. Bonding atoms

C(6)–C(1)–C(2) C(6)–C(1)–C(7) C(2)–C(1)–C(7) C(3)–C(2)–C(1) C(2)–C(3)–C(4) C(5)–C(4)–C(3) C(4)–C(5)–C(6) C(1)–C(6)–C(5) O(1)–C(7)–N(1) O(1)–C(7)–C(1) N(1)–C(7)–C(1) C(7)–N(1)–N(2) C(8)–N(2)–N(1) N(2)–C(8)–C(9) C(10)–C(9)–C(14) C(10)–C(9)–C(8) C(14)–C(9)–C(8) C(11)–C(10)–C(9) C(12)–C(11)–C(10) O(2)–C(12)–C(11) O(2)–C(12)–C(13) C(11)–C(12)–C(13) O(3)–C(13)–C(14) O(3)–C(13)–C(12) C(14)–C(13)–C(12) C(13)–C(14)–C(9) C(13/12)–O(3)–C(15)

3b

acceptors, that is, each molecule of 3a has two hydrogen bond donor atoms and two hydrogen bond acceptor atoms. Hirshfeld analyses

Dihedral angles (Å) 3a

C(6)–C(1)–C(2)–C(3) C(7)–C(1)–C(2)–C(3) C(2)–C(1)–C(6)–C(5) C(7)–C(1)–C(6)–C(5) C(4)–C(5)–C(6)–C(1) C(6)–C(1)–C(7)–O(1) C(2)–C(1)–C(7)–N(1) C(1)–C(7)–N(1)–N(2) C(7)–N(1)–N(2)–C(8) N(1)–N(2)–C(8)–C(9) N(2)–C(8)–C(9)–C(10) C(14)–C(9)–C(10)–C(11) C(8)–C(9)–C(10)–C(11) C(9)–C(10)–C(11)–C(12) C(10)–C(11)–C(12)–O(2) O(2)–C(12)–C(13)–O(3) C(11)–C(12)–C(13)–O(3) O(2)–C(12)–C(13)–C(14) C(11)–C(12)–C(13)–C(14)– O(3)–C(13)–C(14)–C(9) C(12)–C(13)–C(14)–C(9) C(10)–C(9)–C(14)–C(13) C(8)–C(9)–C(14)–C(13) C(14)–C(13)–O(3)–C(15) C(12)–C(13)–O(3)–C(15)

Bond angles (°)

3b

B3LYP

Exp.

B3LYP

0.775 179.356 0.289 179.944 0.093 179.156 179.472 179.670 179.441 179.606 179.857 0.914 179.11 0.877 179.636 0.225 179.665 180.00 0.790 179.619 0.126 0.560 179.462 5.662 174.065

2.2(8) 180.0(5) 2.5(9) 179.9(5) 1.4(12) 142.0(6) 138.4(5) 171.3(4) 173.5(4) 171.8(4) 168.8(4) 0.4(7) 176.1(4) 2.4(8) 177.8(4) 0.7(7) 179.4(4) 179.7(4) 0.4(7) 177.3(4) 1.6(7) 1.5(7) 174.1(4) 3.5(8) 177.5(5)

0.419 179.711 0.396 179.641 0.202 179.521 179.951 179.758 179.972 179.878 179.855 0.748 179.88 0.688 179.956  – 179.965 0.186 – 0.263 0.536 179.965 – –

The crystal packing arrangement and the hydrogen bonding network of 3a are shown in Fig. 2 in different views. Bond lengths and angles related to H-bonding are summarized in Table 4. Sixteen molecules were found to be packed into one unit cell of 3a by intermolecular hydrogen bonding. In which, N(1) and O(2) atoms acted as proton donors and O(1) and N(2) acted as proton

Molecular Hirshfeld surfaces [34] are unique for an individual molecule and provide information on three-dimensional intermolecular interactions in crystals. Hirshfeld surfaces are mapped using the normalized contact distance (dnorm), which is calculated using the following equation

dnorm ¼

di  r vdw de  rvdw e i þ r vdw r vdw e i

where de is the distance from the Hirshfeld surface to the nearest atom outside the surface, di is the distance from the Hirshfeld surface to the nearest atom inside the surface and dnorm is defined in terms of de and di and the van der Waals (vdW) radii of atoms. Three-dimensional (3D) Hirshfeld surface maps are generated with dnorm using a red–white–blue color scheme, indicating shorter contacts, vdW contacts, and longer contacts, respectively, and twodimensional (2D) fingerprint plots generated using de and di. Hirshfeld surface analysis and the associated 2D fingerprint plots of 3a were performed using CrystalExplorer 3.1 software. The Hirshfeld surfaces of compound 3a were mapped over dnorm (0.5 to 1.5 Å) and showed deep red circular depressions visible in front and back surface views indicating hydrogen bonding contacts (Fig. 3). Strong interactions between CAH (C2), OAH (O2), and NAH (N1) and carbonyl O1 atoms manifest in Hirshfeld surfaces as red areas. Some significant p–p interactions were also observed (Fig. 3C) as red and blue triangles, representing p–p stacking. Fig. 4 shows the 2D fingerprint plot of 3a where four distinct spikes indicate different interactions can occur between two adjacent molecules in the crystal structure (Fig. 4). Reciprocal O  H and N  H intermolecular interactions appear as two spikes in the 2D fingerprint plots with 17.8% and 6.0% contributions to total Hirshfeld

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Fig. 2. Packing arrangements in the crystal structure of 3a in different views: (A) b-axis, and (B) Z-clip. Dashed lines indicate hydrogen bonds.

Table 4 Hydrogen-bond geometries (Å, °) in (E)-N0 -(4-hydroxy-3-methoxybenzylidene) benzohydrazide (3a). D–H  A

DH (Å)

H  A (Å)

D  A (Å)

DH  A (°)

O(2)–H(2A)  O(1)i N(1)–H(1A)  N(2)ii

0.84 0.88

2.012 2.723

2.755 3.501

147.03 148.20

Symmetry code: (i) 1=4 + y, 3=4  x, 3=4  z; (ii) ½  x, 1  y, ½ + z

surface, respectively. Whereas, reciprocal H  H interactions, which appeared in the middle of the scattered points in the 2D fingerprint plot, contributed most (43.4%) to total Hirshfeld surfaces, and C  H reciprocal interactions with a contribution of 23.1% provide information on intermolecular hydrogen bonding. However, visible complementary regions in the fingerprint plots showed one molecule acts as donor (de > di) and the other as acceptor (de < di). DFT quantum chemical calculations To investigate the molecular structures and electronic properties of compounds 3a and 3b, quantum mechanical calculations were carried out using grid based density functional theory (DFT) at the B3LYP/6-311G basis set level. The calculated values of bond lengths, bond angles, and dihedral angles for 3a and 3b were compared with experimental values of 3a obtained by X-ray diffraction. Tables 2 and 3 detail the experimental and optimized geometric

parameters of 3a and 3b at the B3LYP/6-311G level using the atom numbering scheme in Figs. 1 and 5. The results obtained showed good agreement between optimized geometric parameters and experimental values. Fig. 5 shows the optimized geometries of the most stable structures of 3a and 3b, which both exhibit C1 symmetry. The total energies of compounds 3a and 3b were 573921.16 and 526675.36 kcal mol1, respectively, which indicate stability of the optimized structures. Selected calculated quantum-chemical parameters of 3a and 3b are presented in Table 5. However, a significant difference was found between the crystal structure and optimized geometry of 3a. In its crystal structure, the benzoyl phenyl ring (C1–C6) was rotated out of the plane formed by the remainder of the molecule at an angle of 39.2(7)° (Fig. 1), whereas it almost lay in the molecular plane in the optimized geometries of 3a and 3b (Fig. 5). We attribute this difference to our determination of the most stable optimized structures of 3a and 3b as isolated molecules in the gas phase where free rotation is possible around the single bonds of N(1)–C(7)– C(1) and C(7)–N(1)–(N2). Therefore, for better insight of the conformational energy changes associated with the N(1)–C(7)–C(1) and C(7)–N(1)–(N2) bonds the potential energy surface scan was performed for different N(1)–C(7)–C(1)–C(6) and C(7)–N(1)– (N2)–C(8) torsion angles for 3a and 3b (Fig. 6). During potential energy calculations N(1)–C(7)–C(1)–C(6) and C(7)–N(1)–(N2)– C(8) torsion angles were varied in increments of 5° from 180° to 180° and all geometric parameters were relaxed. Fig. 6 shows

Fig. 3. 3D Hirshfeld maps with dnorm (front view, A and back view, B), where deep red circle indicating hydrogen bonding contacts, and shape index (C), where red (d) and blue (d+) surfaces show p–p stacking of non-covalent attractive force for compound 3a. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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A

de

B

de

Full (100 %)

O….H (17.8 %)

di D

de

N….H (6.0 %)

di

di

E

de

C….H (23.1 %)

C

de

H….H (43.4 %)

di

di

Fig. 4. 2D fingerprint plots of compound 3a, full (A); showing reciprocal contacts and resolved into: O  H (B), N  H (C), C  H (D) and H  H (E) showing the percentage contact contributing to the total Hirshfeld surface area of the molecule. Black arrows are show spike for different interactions. di is the closest internal distance from a given point on the Hirshfeld surface and de is the closest external contact point.

C10 C5

N2

C6

C4

C7

C8

C12

C14

N1

C10 C11

C9

C3

O2

O3

C2

C5 C6

C4

C13

C1 O1

C11

C9 N2 C7

C2

C12

C14

N1

C13

C1

C3

C15

C8

O2

C15

O1

3b (Keto)

3a (Keto) C10

C3

N2

C6

C4

C7

C8

N1

C14

C12

O1

C10

C5 O2

C13

C1 C2

C11

C9

C5

C4

C6 C7

C3 O3

N2 N1

C9

C11

C8

C1

C14

C12

O1

C15

3a (Enol)

O2

C13

C2

C15

3b (Enol)

Fig. 5. Most stable optimized molecular structure of 3a, 3b and their corresponding enols in the gaseous state calculated using B3LYP/6-311G level of theory.

Table 5 Quantum-chemical properties of 3a, 3b and their corresponding enols calculated using B3LYP/6-311G level of theory. Entry

Kinetic energy

Potential energy

Total energy

Dipole (Db)

EHOMO (eV)

ELUMO (eV)

(kcal Mol1) 3a 3a (enol) 3b 3b (enol)

572545.99 572787.40 526047.66 525571.30

Cp

Cv

Cal Mol1 K1 1146445.81 1146682.94 1052723.03 1052295.01

573921.16 573914.75 526675.36 526669.71

4.25 1.75 3.88 2.36

6.274 6.098 6.187 5.986

1.744 1.902 1.697 1.866

62.72 62.32 50.96 50.45

61.39 60.33 48.98 47.79

EHOMO—energy of the highest occupied molecular orbital. ELUMO—energy of the lowest unoccupied molecular orbital. Cp—Heat capacity at constant pressure. Cv—heat capacity at constant volume.

the torsion angle minimum energy curve and conformational structure of 3a and 3b at a N(1)–C(7)–C(1)–C(6) torsion angle of 39.30°, at which 3a and 3b possessed 75.80 and 66.27 kcal mol1 more energy than in their corresponding optimized structures.

Vibrational frequencies analysis The infra-red vibrational frequencies assignments for compounds 3a and 3b were predicted at the B3LYP/6-311G level at a

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569

95

95 Energy (kcal mol-1 )

85 75 65

85

3b

75 65

55 -180-135 -90 -45 0 45 90 135 180 Torsion angle N(1)-C(7)-C(1)-C(6)

55 -180-135 -90 -45 0 45 90 135 180 Torsion angle N(1)-C(7)-C(1)-C(6)

145

155

125

Energy (kcal mol-1 )

Energy (kcal mol-1 )

Energy (kcal mol-1 )

3a

105 85 65 -180 -120 -60 0 60 120 180 Torsion angle C(7)-N(1)-N(2)-C(8)

135 115 95 75 55 -180 -120 -60 0 60 120 180 Torsion angle C(7)-N(1)-N(2)-C(8)

Fig. 6. The potential energy curves for 3a and 3b along with N(1)–C(7)–C(1)–C(6) and C(7)–N(1)–N(2)–C(8) dihedral angle (°), calculated using B3LYP/6-311G level of theory.

spin multiplicity of one and compared with recorded FT-IR (solid phase) spectra (Table 6 and Fig. 7). No imaginary frequencies were observed among the predicted vibrational frequencies, indicating that the optimized structures were stable, that is, located at local minimum points on potential energy surfaces. As presented in Table 6 and Fig. 7, predicted vibrational frequencies agreed well with experimental values, thus confirming the validities of the optimized structures of 3a and 3b.

obtained by Mulliken population analysis (MPA) using the DFTB3LYP/6-311G method, and results are presented in Table 7. According to MPA analysis, the electro-negativities of bonding atoms play an important role in the distribution of Mulliken charge. The N(1) nitrogen atoms in the ACOANHAN@CHA open chain systems of 3a and 3b bear maximum negative charges of 0.598 and 0.614 a.u., respectively, and the O(2) and O(3) atoms of methoxy and OH groups of also bear high negative charge of 0.617 and 0.549 a.u., respectively. The O(2) atom of the methoxy group of3b possess lower negative charge (0.418) than that of the O(2) of 3a. The N(2) azomethine nitrogens of 3a and 3b also carry lower negative charges of 0.111 and 0.109 a.u., respectively, than that of the amine nitrogen N(1). The carbon atoms C(1)–C(6) of the benzoyl phenyl rings of3a and 3b have negative charges in the range 0.005 to 0.268 a.u. The carbon atoms in the benzylidene phenyl ring C(12) and C(13) of3a, and C(12) of 3b attached to the oxygen atom possess positive charges in the range 0.230–0.265 a.u., whereas the other carbon atoms, that is, C(10), C(11), and C(14), have negative charges in the range0.017 to 0.288 a.u. The methoxy carbon atoms C(15) of3a and 3b carry negative charges of 0.268 and 0.363 a.u., respectively, and as might be expected, the O(1) oxygen atoms and the C(7) carbonyl carbon atoms of3a and 3b carry negative charge in the range 0.367 to 0.378 and positive charge in the range 0.553– 0.597 a.u., respectively. The above results indicate that thehetero atoms with high negative charges, that is, N(1), O(1), O(2), and O(3), could act as electron donors and coordinate with metals which is consistent with the following study [19].

Mulliken charge distributions

Molecular electrostatic potentials

Partial atomic charges can be used to explain the dipole moments, electronic structures, and polarizabilities of molecules, and thus, chemical reactivities. For this reason, calculations of atomic charges are useful component of quantum chemical methods. The charge distributions of all atoms of 3a and 3b were

Electron density distributions over molecular surfaces can be visualized using spatial molecular electrostatic potential (MEP) maps. MEP maps are used to predict chemical reactivities, such as, electrophilic and nucleophilic reactions, in biological systems and hydrogen bonding interactions [35]. To predict the shapes,

Table 6 Comparison of some selected experimental and calculated vibrational wave numbers (cm1) of 3a and 3b at the B3LYP/6-311G level of theory. Assignments

3a

3b

Exp.

Calc.

Exp.

Calc.

OAH str. NAH str. CAH str. >C@O functional group

3430 3231 2919 1648

3499 3258 3083 1670

C@N str. and CAC def. in phenyl ring CAO str. in carbonyl group NAN str. and all CAH def. NAN str. and CAN str. All of the CAH def. Molecular skeleton def. CAC def. in phenyl ring CAH def. in phenyl ring

1599– 1520 1279 1186 1081 852 811 740 682

1613– 1514 1254 1192 1036 893 – 735 687

– 3189 2904 1707– 1647 1596– 1552 1371 1156 1027 915 829 726 691

– 3284 2978 1717– 1654 1588– 1531 1387 1205 1037 954 869 728 706

str. = stretching, def. = deformation.

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Fig. 7. Comparisons of the experimental and theoretical FT-IR spectra of 3a (top) and 3b (bottom). Predicted vibrational frequencies were obtained by DFT-B3LYP/6-311G calculation.

sizes, charge densities, delocalizations, and reactive sites of 3a and 3b, MEPs were calculated for B3LYP/6-311G optimized geometries and crystal structure. Fig. 8 presents the electrostatic potential maps of 3a and 3b, in which blue and red indicate positive and negative potentials, respectively. Because negative regions are targeted by electrophiles, the figure shows there are three possible sites for electrophilic attack in 3a and 3b. In addition, this study

revealed that both compounds are more likely to undergo electrophilic than nucleophilic attack. Frontier molecular orbitals Frontier molecular orbitals (FMOs) also play an important part in determining chemical reactivity, kinetic stability, and the optical

M.S. Alam, D.-U. Lee / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 145 (2015) 563–574 Table 7 Mulliken charges (e) and its population for compounds 3a and 3b. Atoms

C(1) C(2) C(3) C(4) C(5) C(6) N(1) C(7) O(1) N(2) C(8) C(9) C(10) C(11) C(12) C(13) C(14) O(2) O(3) C(15) H(3) H(4) H(5) H(2) H(6) H(N1) H(8) H(10) H(11) H(13) H(14) H(O2) H(15A) H(15B) H(15C)

3a

3b

Charge/e

Population

Charge/e

Population

0.198615 0.124090 0.188289 0.115034 0.152636 0.065605 0.597714 0.595910 0.367351 0.110642 0.012634 0.070000 0.120253 0.153608 0.229590 0.259068 0.165900 0.617678 0.548775 0.268383 0.150753 0.161967 0.154858 0.189943 0.143565 0.321555 0.143169 0.142945 0.170537 – 0.215765 0.380193 0.190191 0.201373 0.200559

6.198615 6.124090 6.188289 6.115034 6.152636 6.065605 7.597714 5.404090 8.367351 7.110642 5.987366 6.070000 6.120253 6.153608 5.770410 5.740932 6.165900 8.617678 8.548775 6.268383 0.849247 0.838033 0.845142 0.810057 0.856435 0.678445 0.856831 0.857055 0.829463 – 0.784235 0.619807 0.809809 0.798627 0.799441

0.267916 0.008541 0.212350 0.086008 0.196428 0.00511 0.613830 0.553422 0.377587 0.108574 0.010812 0.194721 0.016739 0.287947 0.264588 0.198475 0.003353 0.418383 – 0.363523 0.150652 0.153399 0.156520 0.191060 0.140967 0.308815 0.109804 0.154644 0.195296 0.148073 0.154644 – 0.178788 0.211270 0.204624

6.267916 6.008541 6.212350 6.086008 6.196428 5.999489 7.613830 5.446578 8.377587 7.108574 6.010812 6.194721 5.983261 6.287947 5.735412 6.198475 5.996647 8.418383 – 6.363523 0.849348 0.846601 0.843480 0.808940 0.859033 0.691185 0.890196 0.797430 0.804704 0.851927 0.845356 – 0.821212 0.788730 0.795376

properties of molecules, and thus, we calculated the energies of highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular orbitals (LUMO) and their distributions using DFT-B3LYP/6-311G. These calculations identified 34 atoms involving 302 basis functions and 71 occupied orbitals for 3a, and 33 atoms involving 289 basis functions and 67 occupied orbitals for 3b. Nuclear repulsion energies of 3a and 3b were 1358.507 and 1251.932 a.u., respectively. The calculated energy of HOMO and

571

LUMO was6.274 and 1.744 eV for 3a, and 6.187 and 1.697 eV for 3b, respectively. The total energies of 3a (914.601 a.u.), and3b (839.310 a.u.), and the energies of HOMO and LUMO, including neighboring orbitals, were all negative, indicating that both compounds were stable. In a molecule, HOMO is an electron donor and LUMO is an electron acceptor. Furthermore, a low HOMO–LUMO energy gap is known to indicate greater polarizability and chemical reactivity and lower kinetic stability. However, the HOMO–LUMO energy band gap of 3a and 3b were found to be high at 4.53 and 4.49 eV, respectively, indicating both compounds are stable and have high excitation energies. It was observed that the highest occupied molecular orbitals in both compounds are located in the methoxy, hydroxyl, benzylidene phenyl ring, and open chain AC@NANHACOA groups, and that lowest unoccupied molecular orbitals are mainly localized on the benzoyl phenyl ring and open chain AC@NANHACOA groups.

Fluorescence properties Figs. 9 and 10 show the excitation and emission spectra of 3a and 3b, respectively, measured in solvents of different polarities at room temperature. Interestingly, the absorption spectra of 3a in acetone showed hypochromic shifts versus those obtained in ethanol, ethyl acetate, and dichloromethane, and the absorption spectra of 3b in acetone and dichloromethane showed hyperchromic shifts versus those obtained in ethanol and ethyl acetate. A summary of the optical absorption and emission properties of 3a and 3b are presented in Table 8. A substantial energy band gap was observed between the absorption and emission spectra of 3a (Fig. 9) and 3b (Fig. 10). Stokes shifts for emission were calculated to be >8290 and >8984 cm1 for 3a and 3b, respectively. On the other hand, the emission spectrum of 3a in acetone showed hyperchromic shifts versus those in ethanol, ethyl acetate, and dichloromethane, whereas 3b in ethanol also exhibited hyperchromic shifts in acetone versus those in ethyl acetate and dichloromethane. It is interesting to note that both the compounds showed dual emission in acetone at kmax 400–407 and 437–438 nm within the emission band width range of other solvents, i.e. ethanol, ethyl acetate, and dichloromethane, indicating dual functionality, that is, the existence of different molecular entities in excited states bearing different dipole moments and emitting absorbed energies in different ways. A literature survey revealed that hydrazone Schiff bases show keto-enol tautomerism in polar solvents and in transition

Fig. 8. Molecular electrostatic potential map of (A) 3a and (B) 3b calculated by Gamess interface in ChemBio3D ultra 14.0 software, (C) MEP over crystal structure of 3a, and (D) sharp index of the MEP of 3a calculated by Tonto in CrystalExplorer 3.1, using DFT-B3LYP/6-311G level of theory.

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Fig. 9. Normalized excitation (left) and emission (right) spectra of 3a in ethanol (EtOH), acetone (ACT), dichloromethane (DCM), and ethyl acetate (EA).

Fig. 10. Normalized excitation (left) and emission (right) spectra of 3b in ethanol (EtOH), acetone (ACT), dichloromethane (DCM), and ethyl acetate (EA).

Table 8 Summary of the photophysical properties of 3a and 3b in solvents with different polarities.

a b c d

Entry

Solvent

kex (nm)a

kem (nm)a

Stokes shiftb

Uc

Eexd

Eemd

3a

Ethanol Acetone Ethyl acetate Dichloromethane

287 291 297 298

401 400; 438 394 398

9906 9364; 11,533 8290 8432

95 87 94 93

4.32 4.26 4.18 4.16

3.10 3.09; 2.83 3.15 3.11

3b

Ethanol Acetone Ethyl acetate Dichloromethane

290 287 291 291

407 398; 437 394 396

10,013 9718; 11,960 8984 9113

94 83 92 95

4.28 4.32 4.26 4.26

3.05 3.11; 2.84 3.15 3.13

Excitation and emission maxima measured at concentrations of 3.7  106 M for3a and 3.9  106 M for3b. In cm1. Quantum yield (%) calculated by Spekwin32-optical spectroscopy software, version 1.71.6.1. Excitation and emission band gap estimated by using equation E = hC/m.

and inner-transition metal complexes [36,37]. Therefore, the above results led us to speculate that the comparatively polar environment created by acetone might cause some molecules of 3a and 3b to convert to the enol form (Fig. 11) via an excited state intermolecular proton transfer (ESIPT) mechanism and revert to the keto form by ground state intermolecular proton transfer (GSIPT) mechanism. To explain the dual emission of compounds 3a and 3b in acetone quantum mechanical calculations were performed on the enol structures of 3a and 3b using DFT- and time dependent-DFTB3LYP/6-311G level of theory to determined their molecular and electronic properties and energies of their FMOs in ground and excited states. The total energies of the optimized geometries of enol forms of 3a and 3b were 573914.75 and 526669.71 kcal mol1, respectively, which are similar to those of

OCH3 OH

O N H

N

N H

N

OCH3 OH N

3a ( Enol)

N

3b (Keto)

3a (Keto)

OH

OCH3

O

OH N

OCH3 N

3b (Enol)

Fig. 11. Keto-enol tautomerism of 3a (left) and 3b (right).

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573

LUMO (-1.744 eV) LUMO (-1.902 eV) λex = 274 nm f = 0.4291

λex = 295 nm f = 0.2893

HOMO (-6.098 eV) 3a (Enol) Path-II

HOMO (-6.274 eV) 3a (Keto) Path-I

Fig. 12. Proposed mechanism for the excitation and emission spectra of 3a.

LUMO (-1.697 eV) LUMO (-1.866 eV) λex = 276 nm

λex = 301 nm

f = 0.5930

f = 0.3921

HOMO (-5.986 eV) HOMO (-6.187 eV) 3b (Keto)

3b (Enol) Path-II

Path-I Fig. 13. Proposed mechanism for the excitation and emission spectra of 3b.

their keto forms (Table 5). However, dipole moments in the excited states of 3a and 3b and their corresponding enols were 4.09 and 4.36, and 1.88 and 2.65, respectively, which differed from their ground state dipole moments of 4.25 and 3.88, and 1.75 and 2.36D, respectively. It is reported that the change in the magnitudes of dipole moments of excited states is a cause of the solvatochromic effect in fluorescence spectra [38,39]. The HOMO–LUMO contour plots of keto-enol forms of 3a and 3b and their associated energies and the proposed fluorescent mechanisms of 3a and 3b are shown in Figs. 12 and 13, respectively. We propose compounds 3a and 3b in ethanol, ethyl acetate, and dichloromethane utilize Path-I for photoexcitation and emission, whereas in acetone Path-I and Path-II are utilized (Figs. 12 and 13). According to Path-I, upon photoexcitation of 3a and 3b, intermolecular electron density is transferred from the benzylidene phenyl ring and its attached methoxy

groups to the benzoyl phenyl ring. In the excited state, some absorbed energy is lost by non-radiative transfer and remaining energy is emitted at longer wavelength due to electron transition from LUMO to HOMO (Path-I). However, for Path-II, after photoexcitation some molecules convert to their enol forms by intermolecular proton transfer from N(1) to O(1) of the carbonyl group. As a result, electron density in the benzylidene phenyl ring increases in the LUMO of the enol form due to the formation of a conjugated system as compared with that of the LUMO of the keto form. Therefore, the energy level of the LUMO decreases from 1.744 and 1.697 eV to 1.902 and 1.866 eV for 3a and 3b, respectively, indicating that the formation of the excited state enol tautomer is thermodynamically favorable. Upon photoemission from LUMO to HOMO of enol tautomers, electron densities concentrate around the AC(OH)@NAN@CA open chain and methoxy

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Table 9 Calculated electronic excitation energies, corresponding oscillator strengths (f), and main configurations for 3a, 3b and their corresponding enols.

a

Entry

Singlet

Electronic transition

Energy

f

Compositiona

3a 3a (enol) 3b 3b (enol)

UV–vis UV–vis UV–vis UV–vis

S ? S1 S ? S1 S ? S1 S ? S1

4.53 eV/274 nm 4.20 eV/295 nm 4.49 eV/276 nm 4.12 eV/301 nm

0.4291 0.2893 0.5930 0.3921

H?L H?L H?L H?L

H stands for HOMO and L stands for LUMO. Only main configurations are presented.

group of 3a and around the AC(OH)@NAN@CA open chain of 3b, which results in increases in HOMO energies of enol tautomers by values of 0.176 and 0.201 eV for 3a and 3b, respectively, compare to that of their corresponding keto tautomers. Full geometry optimization based on the DFT-B3LYP/6-311G level of theory reveals that the keto tautomers of 3a and 3b in the ground state are more stable than the corresponding enol tautomers by 6.41 and 5.65 kcal mol1, respectively. The calculated absorption and emission energy band gap of 3a and 3b, and of their corresponding enols are presented in Table 9, and all agree reasonably well with experimental data (Table 8). However, the calculated absorption spectra of enol tautomers show significant red shift compare to that of their corresponding keto tautomer.

References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]

Conclusions We report the synthesis of two benzoic acid hydrazide Schiff base analogues, that is, (E)-N0 -(4-hydroxy-3-methoxybenzylidene)benzohydrazide (3a) and (E)-N0 -(4-methoxybenzylidene)benzohydrazide (3b) and their molecular structures, Hirshfeld surfaces, vibrational frequencies, coordinations, and fluorescence spectral properties. X-ray diffraction analysis of a single crystal of 3a revealed a tetragonal space group I4(1)/a and an Econfiguration around the azomethine (AC8@N2A) double bond. Sixteen molecules were found to be packed into one unit cell by weak non-classical intermolecular NAH  N and OAH  O hydrogen bonds. Hirshfeld analyses revealed close O—H, N—H, C—H, and H—H contacts and p–p stacking in the crystal structure. Vibrational frequency analysis showed good agreement between solid state (experimental) and gaseous phase (calculated) modes of vibration. Mulliken charge distribution analysis revealed that both compounds possess significant coordination sites, and N(1), O(1), O(2), and O(3) were found to be potential electron donating sites for coordination with metals. However, molecular electrostatic potential surface maps showed O(1), N(1)–N(2), and O(2) and/or O(3) are the most suitable sites for electrophilic attack and that both compounds are more likely to undergo electrophilic than nucleophilic attack. Regarding fluorescence spectra, both compounds showed similar absorption and emission spectra. Interestingly, compounds 3a and 3b both showed dual emission in acetone originating from keto-enol tautomerism upon photoexcitation. Appendix A. Supplementary material Crystallographic data for compound 3a has been deposited at the Cambridge Crystallographic Data Center (Deposition number CCDC 1020312). Data can be obtained (free of charge) from: the Cambridge Crystallographic Data Center (CCDC), 12 Union Road, Cambridge, CB2 1EZ, UK; fax: +44(0) 1222-336033; e-mail: [email protected]; www.ccdc.cam.ac.uk/Community/ Requestastructure/Pages/DataRequest.aspx

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