Spectral and crystal studies on 5-(ethoxycarbonylmethoxy)-9-(phenylazo)benzaldoxime: DFT approach

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 125 (2014) 328–334

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Spectral and crystal studies on 5-(ethoxycarbonylmethoxy)9-(phenylazo)benzaldoxime: DFT approach R. Balachander ⇑ Department of Chemistry, Achariya College of Engineering Technology, Villianur 605 110, Puducherry, India Department of Chemistry, Annamalai University, Annamalainagar 608 002, Tamilnadu, India

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

 2-(Ethoxycarbonylmethoxy)-5-

(phenylazo)benzaldoxime was synthesized and characterized.  Conformation analyzed by spectral and computational techniques.  Single crystal measurements also support the same above conformation.

a r t i c l e

i n f o

Article history: Received 23 August 2013 Received in revised form 20 December 2013 Accepted 8 January 2014 Available online 21 January 2014 Keywords: 5-(Ethoxycarbonylmethoxy)-9(phenylazo)benzaldoxime Spectral Computational Crystal studies

a b s t r a c t 5-(Ethoxycarbonylmethoxy)-9-(phenylazo)benzaldoxime was synthesized from arylazosalicylaldehyde and characterized by IR, 1H and 13C NMR spectroscopy. The crystal structure of title compound was determined by X-ray crystallography. Single crystal data reveal trans configuration of aromatic rings about N@N bond and the two rings are nearly coplanar and oxime group adopts E configuration. The conformation of title compound was determined theoretically and NAO rotational barrier was also computed theoretically. HOMO–LUMO energies polarizability, hyperpolarizability, natural bond orbital (NBO), atom in molecule (AIM) analysis and atomic charges were also calculated theoretically according to density functional theory (DFT) method and analyzed. Ó 2014 Elsevier B.V. All rights reserved.

Introduction Azo compounds are the oldest and largest class of industrial synthesized organic dyes due to their versatile application in various fields, such as dyeing textile fiber, biomedical studies, advanced application in organic synthesis and high technology areas such as laser, liquid crystalline displays, electro-optical devices and ink-jet printers [1–3]. The pharmaceutical importance ⇑ Address: Department of Chemistry, Achariya College of Engineering Technology, Villianur 605 110, Puducherry, India. Tel.: +91 9344790744. E-mail address: [email protected] 1386-1425/$ - see front matter Ó 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2014.01.037

of compounds containing an arylazo group have been extensively reported in the literature [4,5]. The oxidation–reduction behavior of these compounds play an important role in their biological activity [6]. The preparation of new ligands is perhaps the most important step in the development of metal complexes with unique properties and novel reactivity [7]. Oximes and azo dyes have often been used as chelating ligands and the biological importance is also very well known [8]. Different oximes and their metal complexes have shown notable bioactivity as chelating therapeutics, as drugs as inhibitors of enzymes and as intermediates in the biosynthesis of nitrogen oxides [9,10]. The present investigation focuses on the synthesis and theoretical investigation of the molecular

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structure, charges, NBO and AIM analysis of 2-(ethoxycarbonylmethoxy)-5-(phenylazo)benzaldoxime. HOMO–LUMO energies, dipole moment, polarizability and first hyperpolarizability were also determined by DFT method and analyzed.

Experimental

329

Results and discussion 5-(Ethoxycarbonylmethoxy)-2-(phenylazo)benzaldoxime was synthesized as shown in Scheme 1 and characterized by 1H, 13C, 1 H–1H and 1H–13C COSY spectra. The single crystal measurement was also recorded for this compound. The labelling of the atoms followed in the present study was indicated in Scheme 1.

Synthesis of 5-(ethoxycarbonylmethoxy)-9-(phenylazo)benzaldoxime NMR spectral analysis The starting material 5-(ethoxycarbonylmethoxy)-9-(phenylazo)benzaldehyde was prepared by stirring a mixture of 9-phenylazosalicylaldehyde [11] (0.226 g, 1 mmol), ethylbromoacetate (0.11 mL, 1 mmol) and potassium carbonate (0.5 g) in acetonitrile (15 mL) for 24 h. The reaction mixture was filtered and the solvent was evaporated to obtain 5-(ethoxycarbonylmethoxy)-9-(phenylazo)benzaldehyde. The pure product was obtained by recrystallization from ethanol. Yield: 70%; m.p. 86 °C. A mixture of 5-(ethoxycarbonylmethoxy)-9-(phenylazo)benzaldehyde (0.312 g, 1 mmol) and sodium acetate (0.5 g) was dissolved in boiling ethanol and hydroxylamine hydrochloride (0.13 g, 2 mmol) was added. The mixture was refluxed for 3 h. The reaction mixture was poured into water. The 5-(ethoxycarbonylmethoxy)9-(phenylazo)benzaldoxime separated out was filtered and recrystallized from ethanol. Yield: 70%, m.p. 153 °C. IR (KBr, cm 1): 1605 (mC@N), 1752 (mCOO), 3251 (mOH), 1316 (mN@N) and 1218 (dOH). Spectral measurements The 1H NMR (500 MHz) and 13C NMR (125 MHz) were recorded at room temperature on Bruker 500 MHz instrument using 10 mm sample tube. Samples were prepared by dissolving about 10 mg of the sample in 2.5 mL of chloroform-d containing 1% TMS for 1H and 50 mg of the sample in 2.5 mL of chloroform-d containing a few drops of TMS for 13C. The solvent chloroform-d also provided the internal field frequency lock signal. The 1H–1H and 1H–13C COSY spectra were performed on a Bruker 500 NMR spectrometer.

Computational study Geometry optimizations were carried out according to density functional theory available in Gaussian-03 package using B3LYP/ 6-31G(d,p) basis set [12] and also according to MP2 method using the [CHELPG] basis set 6-31G(d) available in Gaussian-03 package. The polarizabilities and hyperpolarizabilities were determined from the DFT optimized structure by finite field approach using B3LYP/6-31G basis set, NBO calculations using the basis set B3LYP/6-311+G(d,p) available in Gaussian-03 and AIM calculations were done using B3LYP/6-31G(d,p) basis set. Charges, were also calculated according to MP2 method using the [CHELPG] basis set 6-31G(d) available in Gaussian-03 package.

X-ray analysis A single crystal of title compound with the dimensions 0.30  0.25  0.20 mm was chosen for X-ray diffraction study. Crystallographic measurements were done at 293(2) K with Bruker axis Kappa CCD diffractometer using graphite monochromated Mo Ka radiation (k = 0.71073 Å). The crystal structure was solved by direct method and refined by full-matrix least square technique on F2 using the SHELX-97 set of program [13]. The parameters in the CIF form are available as electronic supplementary information from the Cambridge Crystallographic Database Centre [CCDC 846567].

The signals in the 1H NMR spectrum (Fig. S1) were assigned based on their positions, integrals and multiplicities. The triplet and quartet appeared at 1.31 and 4.30 ppm are due to methyl and methylene protons of the ethyl group attached to the COO moiety. A singlet observed at 4.81 ppm is corresponding to methylene proton attached to the oxygen atom O–15. The singlet exhibit at 8.64 ppm is due to the H–7 proton. The downfield doublet resonated at 6.93 ppm is corresponding to the ortho proton with respect to OCH2COO moiety i.e., H–11. The doublet seems at 8.37 ppm (integral corresponds to one proton) and doublet of doublet observed at 7.97 ppm (integral corresponds to one proton) are could be attributed to the ring proton H–8 and H–10 respectively. The doublet nature of the signal for H–8 is due to meta coupling (J8,10 = 5.00 Hz) and this coupling is also observed in the signal for H–10 which appears as doublet of doublet due to J8,10 (5.00 Hz) and J9,10 (10.00 Hz). The 1H NMR spectrum further reveals that the downfield doublet at 7.89 ppm for ortho protons of the phenyl ring attached to the nitrogen atom N–3 i.e., (H–13 and H–17). Two triplets at 7.50 (integral corresponds to two protons) and 7.47 ppm (integral corresponds to one proton) are also observed for meta (H–14 and H–16) and para (H–15) protons of the phenyl ring attached to nitrogen atom N–3. This assignment is further confirmed by the correlations observed in the 1H–1H COSY spectrum as shown in Fig. 1. In 13C NMR spectrum (Fig. S2), the signals at 14.2 and 61.7 ppm are due to the methyl and methylene carbon of ethyl group attached to oxygen atom O–1. The methylene carbon attached to oxygen O–3 resonates at 66.1 ppm. The low intense signal at 168.12 ppm is due to ester carbonyl carbon C–3 [14]. The signal at 112.5 ppm is corresponding to ortho carbon with respect to oxygen atom O–3 i.e., C–11. The high intense signals at 123.0 and 129.0 ppm are should be attributed to ortho (C–13 and C–17) and meta (C–14 and C–16) carbons of the phenyl ring attached to nitrogen atom N–3. The 13C NMR spectrum reveals four signals for quaternary carbons (C–6, C–5, C–9 and C–12) at 123.1, 157.7, 147.3 and 152.4 ppm [15–20] which can easily be distinguished from other carbons based on small intensities. Among the signals for quaternary carbons, the downfield signal at 157.7 ppm is due to the ipso carbon C–5 and this conformation is based on the deshielding nature of the oxygen atom compared to nitrogen and carbon atoms. The signal at 123.1 ppm is assigned to ipso carbon C–6. Furthermore, the signal at 152.4 ppm is assigned to the ipso carbon C–12 which is attached to the nitrogen atom N–3. Obviously, the signal at 147.3 ppm is assigned to the carbon C–9 which is para with respect to the OCH2 moiety. The spectrum further reveals four signals at 146.0, 130.8, 126.3 and 121.8 ppm. Among these signals the high frequency signal (146.07 ppm) is obviously due to carbon attached to NOH group (C–7). The signals at 121.8 and 126.3 ppm are due to ortho carbons (C–8 and C–10) of phenyl ring with respect to nitrogen atom N–2. The para (C–15) carbon of the phenyl ring attached to the nitrogen atom N–3 resonate at 130.8 ppm. This assignment is further confirmed by the correlations observed in the 1H–13C COSY spectrum as shown in Fig. 2. Table 1 lists the NMR data of oxime.

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Scheme 1. Steps involved in the synthesis of oxime.

Fig. 1. 1H–1H COSY spectrum of oxime.

Conformational analysis There are four possible conformations for the title compound as shown in Scheme 2. The oxime moiety adopts E configuration in conformations A and C and Z configuration in conformations B and D. The conformers A and C differ in the conformation of COOCH2CH3 group only, so also conformers B and D. The carbonyl oxygen of ethoxycarbonyl group i.e., O–2 is on the same side as that of alkoxy oxygen O–3 in conformations A and C whereas it is on opposite side in conformations B and D. It is seen from Scheme 2 that steric interaction exists between oxime moiety and alkoxy moiety in conformations C and D and hence not favoured in the present study. Further E configuration of oxime is favoured over the Z configuration generally and hence the favoured conformation is predicted to be A. In order to confirm the favoured conformation computational calculations were performed according to DFT method using B3LYP/6-31G(d,p) basis set available in Gaussian-03 pack-

age [12] and the relative energies determined are found to be 0.0 (A), 1.757 (B), 0.126 (C) and 1.945 (D) kcal mol 1. Thus, the theoretical study predicts the favoured conformation as A (E configuration) only. Potential energy scan over NAO bond in E configuration was also carried out to predict the orientation of OH group of oxime moiety in conformation A. The torsional angle C(7)AN(1)AO(4)AH(4) was varied in steps of 10° from 0° to 180° and potential energy scan diagram thus obtained is illustrated in Fig. 3. From the scan diagram it is concluded that OAH bond is anti to C@N bond [C(7)AN(1)AO(4)AH(4) = 180°] and this arrangement is favoured the syn arrangement (torsional angle = 0°) and the barrier for this process is determined to be 8.227 kcal mol 1 (0.0131 Hartree). From the favoured structure selected geometric parameters were derived. The optimized structure of the favoured confirmation of oxime is shown in Fig. 4. Single crystal measurements were also made for the oxime and it also supports the same conformation. The ORTEP structure and close packed diagram are shown in Fig. 5.

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331

Fig. 2. 1H–13C COSY spectrum of oxime.

Table 1 H and 13C NMR chemical shifts (ppm) of oxime.

1

1

13

H chemical shift

Proton

d

H–11 H–10 H–8 H–7 H–4 H–2 H–1 H–13 and H–17 H–14 and H–16 H–15

6.93 7.97 8.37 8.64 4.81 4.30 1.31 7.89 7.50 7.47

(d, 10.00 Hz) (dd) (d, 5.00 Hz)

(q, 10.00 Hz) (t, 10.00 Hz) (d, 7.50 Hz)

C chemical shift

Carbon

d

C–11 C–10 C–8 C–7 C–4 C–3 C–2 C–1 C–6 C–5 C–9 C–12 C–13 and C–17 C–14 and C–16 C–15

112.5 126.3 121.8 146.0 66.1 168.1 61.7 14.2 123.1 157.7 147.3 152.4 123.0 129.0 130.8

XRD analysis A summary of crystallographic data, experimental details and refinement results for title compound are given in Table 2. The aromatic rings adopt a trans configuration about the N@N double bond and are nearly coplanar with a dihedral angle of 179.2°. A weak intramolecular interaction involving oxime OH and azo nitrogen N2 (O4AH4


N2) i.e., hydrogen bond was also observed and the values are also listed in Table 2 itself. Molecular properties Geometric parameters From the optimized geometrical parameters were derived from DFT and MP2 method (Table S1). The selected XRD and computed bond distances, bond angles and torsional angles for title compound. The theoretical bond lengths and bond angles are closer to the XRD values. However, for the torsional angles deviation is

Scheme 2. Possible conformations of oxime.

noticed and the maximum deviation in torsional angles is found to be 7°. The small differences between the theoretical and experimental parameters can be attributed to the fact that the DFT

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R. Balachander / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 125 (2014) 328–334 Table 2 Crystal data and structural (phenylazo)benzaldoxime.

Fig. 3. Potential energy scan diagram of oxime.

refinement

Empirical formula Formula weight Temperature (K) Wave length (Å) Crystal system Space group a (Å) b (Å) c (Å) a (°) b (°) c (°) Volume (Å3) Z Dx (g cm 3) l (mm 1) F (0 0 0) Crystal size h Range (°) Index ranges Reflections collected Independent reflections Absorption correction Max. and min. transmission Refinement method Data/restraints/parameters Goodness-of-fit on F2 Final R indices (all data) R indices [I > 2r(I)] Largest diff. peak and hole (e Å

3

)

for

5-(ethoxycarbonylmethoxy)-9-

C17H17N3O4 327.34 293(2) 0.71073 Monoclinic P-21/c 7.4640(11) 22.034(3) 10.5312(17) 90 108.709(4) 90 1640.4(4) 4 1.325 0.096 688 0.30  0.25  0.20 mm 1.85–27.50 5 6 h 6 9; 28 6 k 6 28; 13 6 l 6 13 18,653 3757 Semi-empirical from equivalents 0.9810 and 0.9717 Full-matrix least-squares on F2 3758/0/219 1.055 R1 = 0.0508, wR2 = 0.1602 R1 = 0.0702, wR2 = 0.1832 0.510 and 0.274

Hydrogen-bonding geometry (Å) of oxime DAH


A O4AH4


N2 x + 1,

Fig. 4. Optimized structure of oxime.

y,

O4AH4 0.82

H4


N2 2.12

O4


N2 2.935(2)

O4AH4


N2 170.6

z + 1.

calculation was carried out with isolated molecule in the gaseous phase, whereas the X-ray parameters were based on molecules in the solid state. Energies, dipole moment and polarizabilities HOMO–LUMO energies, electronic dipole moment, polarizabilities and hyperpolarizabilities were also derived and the values are listed in Table 3 and the HOMO–LUMO plots are given in Fig. 6. The HOMO orbital is mainly derived from pz orbitals of carbon, nitrogen and oxygen atoms except those of COOCH2CH3 group [C3, O2, O1, C2 and C1]. The formation of LUMO orbital does not involve the participation of pz orbitals of carbon (C14), carbon (C7) and oxygen (O4) of CH@NOH group and CH2COOCH2CH3 group [C4, C3, O2, O1, C2 and C1]. Table 3 HOMO–LUMO energy (eV), dipole moment l (D), polarizability and hyperpolarizability of oxime. HOMO–LUMO LUMO

Dipole moment

HOMO

2.08

5.73

DE

lx

3.65

1.62

Polarizability

axx axy ayy axz ayz azz atot (esu)  1024

Fig. 5. ORTEP and close packed structures of oxime.

ly 2.59

lz

ltot

0.23

3.06

Hyperpolarizability 418.0532 0.0415 89.7557 58.8608 0.0555 276.1727 38.7228

bxxx bxxy bxyy byyy bxxz bxyz byyz bxzz byzz bzzz btot (esu)  1035

2005.8278 3.8112 10.3233 2.2064 785.7970 1.1818 58.9065 494.7646 1.2076 20.6832 225.0367

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delocalized on to the pure non-bonding p-orbitals available on C–9 carbon atom, which in turn delocalized on to the nearby antibonding orbitals of the vicinal C6AC8, N2AN3 and C10AC11 bonds. This hyperconjugative interactions energy is found to be very high (67.20, 73.50 and 73.95 kcal mol 1) and this is the primary delocalization seen in the oxime. AIM analysis We have calculated at DFT level the charge density q(r) Laplacian of q(r) [D2q(r)] and ellipticity (e) at the position of the bond critical points and charge density of the ring critical point in oxime (Table S2). Atoms in molecules electron density topological analysis revealed the existence of 41 bcp with a (3, 1) topology between the atoms connected by covalent bond. The negative values obtained for the Laplacian except for C@O group are a clear indication that the corresponding bonds are covalent bonds. Besides these one more bond critical point is observed between C3 and O2 atoms (ester carbonyl group). The positive Laplacian of the density at the BCP and high qBCP values for carbonyl group indicate the ionic nature of the carbonyl group. Ring critical point of phenyl ring attached to N–3 is slightly having higher electron density compared to the phenyl ring of the benzaldoxime moiety.

Fig. 6. HOMO–LUMO pictures of oxime.

NBO analysis NBO analysis at B3LYP/6-311+G(d,p) level were carried out for the oxime and the important second order perturbative estimates of donor-acceptor interactions are displayed in Table 4. The occupancies and the energies of the orbitals involved in primary delocalization are also reported in Table 4. The interaction between filled and empty NBO’s can be described as a hyperconjugative electron transfer process from the donor (filled) to the acceptor (vacant) orbital and the energy lowering due to this interaction is expressed as E2. The p-bonded electrons of C6AC8 bond are

Charges Charges calculated from NBO calculations and MP2 method [CHELPG 6-31G(d) as basis set] (Table S3). From Table S3 it is inferred that all hydrogens and sp2 carbons attached to electronegative atoms such as oxygen and nitrogen [C–5, C–9, C–12, C–7 and C–3] alone attain positive charge according to both the methods. However for sp3 hybridized carbons attached to electronegative atom oxygen [C–4 and C–2] the sign of charge predicted by MP2 method (positive charge) is opposite to that of NBO method (negative charge). Conclusions Structure of 5-(ethoxycarbonylmethoxy)-9-(phenylazo)benzaldoxime was analyzed by NMR, single crystal measurement and computational techniques. Oxime group adopts E configuration

Table 4 Energies, occupancies of primary donor and acceptor orbitals and their delocalization energies of oxime. Donor NBO BD(2)C6AC8 BD(2)C6AC8 BD(2)C6AC8 BD(2)C11AC10 BD(2)C11AC10 BD(2)N2AN3 BD(2)N2AN3 BD(2)C13AC14 BD(2)C13AC14 BD(2)C12AC17 BD(2)C12AC17 BD(2)C12AC17 BD(2)C15AC14 BD(2)C15AC14 LP(1)C5 LP(1)C5 LP(1)C9 LP(1)C9 LP(1)C9 LP(2)O3 LP(1)O2 LP(2)O2 LP(2)O2 LP(2)O1 LP(2)O4

Occupancy 1.64084 1.64084 1.64084 1.70583 1.70583 1.91150 1.91150 1.67460 1.67460 1.61512 1.61512 1.61512 1.65995 1.65995 0.96579 0.96579 1.04304 1.04304 1.04304 1.83460 1.97871 1.83683 1.83683 1.79626 1.89819

Energy (a.u.) 0.25420 0.25420 0.25420 0.26446 0.26446 0.36446 0.36446 0.25152 0.25152 0.25028 0.25028 0.25028 0.25227 0.25277 0.13283 0.13283 0.11451 0.11451 0.11451 0.33454 0.70101 0.27465 0.27465 0.34183 0.31587

Acceptor NBO 

LP (1)C5 LP((1)C9 BD(2)C7AN1 LP(1)C5 LP(1)C9 LP(1)C9 BD(2)C12AC17 BD(2)C12AC17 BD(2)C15AC16 BD(2)N2AN3 BD(2)C14AC13 BD(2)C15AC16 BD(2)C14AC13 BD(2)C12AC17 BD(2)C6AC7 BD(2)C11AC10 BD(2)C6AC7 BD(2)C11AC10 BD(2)N2AN3 LP(1)C5 RY(1)C3 BD(1)C4AC3 BD(1)C3AO1 BD(2)C3AO2 BD(2)C7AN1

Occupancy 0.96579 1.04304 0.15597 0.96579 1.04304 1.04304 0.37241 0.37241 0.32614 0.21747 0.28890 0.32614 0.28890 0.37241 0.35384 0.31702 0.35384 0.31702 0.21747 0.96579 0.01930 0.07394 0.09849 0.21391 0.15597

Energy 0.13283 0.11451 0.01971 0.13283 0.11451 0.11451 0.03130 0.03130 0.02981 0.02188 0.03531 0.02981 0.03531 0.03130 0.03104 0.02257 0.03104 0.02257 0.02188 0.13283 0.91380 0.33961 0.36094 0.00270 0.01971

E2 (kcal mol 65.25 45.13 15.85 58.12 38.87 17.66 10.58 19.65 20.87 20.30 19.21 19.52 18.55 21.08 51.02 53.59 73.50 73.95 67.20 54.21 15.33 20.98 31.07 47.18 17.80

1

)

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and NAO rotational barrier was computed to be 8.227 kcal mol 1. Several molecular properties were determined from optimized structure. Theoretically derived geometric parameters are compared with the experimentally determined values. Acknowledgement The authors thank the NMR Research Centre, Indian Institute of Science, Bangalore for recording the NMR spectra. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.saa.2014.01.037. References [1] S.C. Catino, E. Farris, Concise Encyclopedia of Chemical Technology, John Wiley and Sons, New York, 1985. [2] K. Venkataraman, The Chemistry of Synthetic Dyes, Academic Press, New York and London, 1970 (Chapter VI). [3] R. Egli, A.P. Peter, H.S. Freeman, Colour Chemistry: The Design and Synthesis of Organic Dyes and Pigments, Elseiver, London, 1991 (Chapter VII). [4] H.G. Garg, R.A. Sharma, J. Med. Chem. 12 (1996) 1122. [5] E.J. Modest, H.N. Schlein, G.E. Foley, J. Pharmacol. 9 (1957) 68. [6] L.K. Ravindranath, S.R. Ramadas, S.B. Rao, Electrochim. Acta 28 (1983) 601.

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