Synthesis, experimental and theoretical characterization of N,N′-dipyridoxyl (1,4-butanediamine) Schiff-base ligand and its Cu(II) complex

Spectrochimica Acta Part A 78 (2011) 1046–1050

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Synthesis, experimental and theoretical characterization of N,N -dipyridoxyl (1,4-butanediamine) Schiff-base ligand and its Cu(II) complex Hossein Eshtiagh-Hosseini a,∗ , Mohammad R. Housaindokht a , S. Ali Beyramabadi b , S. Hamid Mir Tabatabaei a , Abbas Ali Esmaeili c , Malihe Javan Khoshkholgh c a b c

Department of Chemistry, Ferdowsi University of Mashhad, Mashhad, Iran Department of Chemistry, Faculty of Science, Islamic Azad University, Mashhad Branch, Mashhad, Iran Department of Chemistry, Faculty of Science, Birjand University, Birjand, Iran

a r t i c l e

i n f o

Article history: Received 12 February 2010 Received in revised form 12 December 2010 Accepted 14 December 2010 Keywords: Schiff-base Salen Copper IR assignment NMR DFT

a b s t r a c t A new N,N -dipyridoxyl(1,4-butanediamine) [=H2 BS] Schiff-base ligand and its Cu(II) salen complex, [Cu(BS)(H2 O)(CH3 OH)], were synthesized and characterized by IR, UV–vis, 1 H NMR, mass spectrometry and elemental analysis. Also, full optimization of the geometries, 1 H NMR chemical shifts (for the H2 BS) and vibrational frequencies were calculated by using density functional theory (DFT) method. Structure of the H2 BS ligand is not planar, i.e. two pyridine rings are not in the same plane. In the structure of the Cu complex, the Schiff-base ligand acts as a dianionic tetradentate ligand in N, N, O− , O− manner. The coordinating atoms of BS2− occupy equatorial positions of the octahedral complex, where the H2 O and CH3 OH ligands locate at axial positions. The calculated results are in good agreement with the experimental data, confirming the suitability of the proposed and optimized structures for the H2 BS ligand and its Cu complex. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Because of the catalytic [1–3] and biological [4–6] activities, synthesis and characterization of salen complexes is of great importance. Coordination of Schiff-base ligands to metal ions improves their biological activities [7,8]. Copper-Schiff-base complexes have been used as catalysts for oxidation of alcohols [9,10]. Also, biological activities of these complexes have been widely studied, especially the DNA-binding and DNA-cleaving properties [11–13] and mimic of galactose oxidase [14]. Biological activity of the copper Schiff-base complexes with 1,4butanediamine bridge has been previously determinated [15–17]. Hopping to biological and catalytic applications, we report here, synthesis and characterization of the H2 BS Schiff-base ligand and its Cu(II) complex, where the H2 BS is N,N -dipyridoxyl (1,4butanediamine), by spectroscopic approaches. Also, optimized geometries of the ligand and Cu complex, theoretical assignment of the IR and 1 H NMR spectra have been calculated by using DFT method. By comparison between the theoretical and experimental results, validity of the proposed structures for the ligand and complex will be evaluated. Determinated structural parameters of the

ligand and its Cu(II) complex, and assignment of the IR bands and 1 H NMR chemical shifts can be used as databases for identification of similar compounds, especially the salens with 1,4-butanediamine bridge. 2. Experimental 2.1. Materials and methods All of used chemicals and solvents were obtained from Merck except for pyridoxal hydrochloride which obtained from Fluka. They were used without further purification. Melting points were measured by using an electrothermal 9100 melting point apparatus. Elemental analysis (C, H, N) was obtained using a Heraeus elemental analyzer CHN-O-Rapid. IR spectra were measured on a Perkin-Elemer 783 infrared spectrophotometer and electronic spectra were recorded on a Shimadzu UV-vis 2500 spectrometer. Mass spectra of the H2 BS ligand and Cu complex were recorded on a Shimadzu-GC-Mass-Qp 1100 Ex and Varian mat CH-7 mass spectrometers, respectively. 1 H NMR spectra were obtained on Bruker Drx-500 Avance spectrometer (500.13 MHz), with (CD3 )2 CO as a solvent. 2.2. Synthesis of H2 BS ligand

∗ Corresponding author. Tel.: +98 511 8797022; fax: +98 511 8796416. E-mail address: [email protected] (H. Eshtiagh-Hosseini). 1386-1425/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2010.12.045

Pyridoxal hydrochloride (611 mg, 3 mmol) was dissolved in 5 mL of methanol, and then was added to methanolic NaOH (303 mg,

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Fig. 1. Structure and B3LYP optimized geometry of the N,N -dipyridoxyl (1,4-butanediamine) together with its labeling.

3 mmol in 5 mL). This mixture was stirred for a few minutes. A solution of 1,4-diaminobutane (243 mg, 1.5 mmol in 5 mL) was slowly added to the mixture. The mixture was stirred for 5 h. The yellow solid was filtered, washed with cold methanol and dried in air (yield: 72%, m.p.: 173). Anal. Calc. for C20 H26 N4 O4 : C, 62.18; H, 6.74; N, 14.51. Found: C, 62.10; H, 6.79; N, 15.47%. 2.3. Synthesis of [Cu(BS)(H2 O)(CH3 OH)] complex The H2 BS Schiff-base ligand (23 mg, 0.06 mmol) together with NaOH (5 mg, 0.12 mmol) was dissolved in 13 mL methanol. Then a solution of Cu2 (CH3 COO)4 ·2H2 O (12 mg, 0.06 mmol) in 6 mL methanol was added dropwise to the ligand solution. The mixture was stirred for 90 min at room temperature. After filtering, the filtrate was washed and dried in 50 ◦ C (yield: 70%). Anal. Calc. for CuC21 H30 N4 O6 : C, 50.60; H, 6.02; N, 11.24. Found: C, 51.58; H, 5.73; N, 12.27%. 2.4. Computational details All calculations reported in this paper were obtained by using gradient-corrected DFT method with the B3LYP functional [18] as implemented in the Gaussian 98 program package [19]. Geometries of the H2 BS ligand and its Cu complex were optimized at 6-31G(d, p) basis sets expect for Cu atom where LANL2DZ basis sets were used. The 1 H NMR chemical shifts of H2 BS were predicted at the same computational level, with respect to tetramethylsilane (TMS). The GIAO method was used for prediction of DFT nuclear shieldings [20]. The harmonic vibrational frequencies on the fully optimized geometries of H2 BS ligand and its Cu(II) complex were performed at B3LYP level, where the used basis sets are LANL2DZ for Cu and 631G basis sets for the other atoms. Due to reasons such as the use of finite basis sets and incomplete treatment of electron correlation, the DFT vibrational frequencies are usually higher than the experimental ones, which can be corrected by applying the procedure of scaling the wavenumbers [21]. Here, the scale factor of 0.9614 was used for calculated wavenumbers [22]. 3. Results and discussion 3.1. Geometry optimization Some experimentally structural data are available for the Schiffbase ligands [23–25] and especially Cu Salen complexes with 1,4-butanediamine N(CH2 )4 N bridge [26–30]. Therefore, determination of structural parameters of the H2 BS ligand and its Cu(II) complex is of great importance. Their optimized structures with labeling of their atoms are shown in Figs. 1 and 2, respectively. In the optimized structure of the H2 BS, two substituted pyridine rings are not in the same plane, but make a dihedral angle of approximately 70.0◦ to each other (Fig. 1). The calculated C3–C1–C9–C11

and C2–C4–C9–C11 dihedral angles are 72.6◦ and 69.6◦ , respectively. The pyridine rings are essentially planar and the bond distances of C C (138.9–141.7 pm) and C N (133.0–14.6 pm) bonds in these rings are in the expected range [31]. The aromatic hydrogens (H3 and H11) are essentially in the same plane with the pyridine rings, where the calculated H3–C3–N3–C2 and H11–C11–N4–C10 dihedral angles are −179.8◦ and 179.2◦ , respectively. Also, the substituted –CH3 and –CH2 OH groups are in the same plane with the pyridine rings. The dihedral angles are about 179.8◦ . The calculated bond lengths of C17–N1 (145.6 pm) and C20–N2 (145.7 pm) are appropriate sizes for the single C–N bond, but the C8–N1 (128.6 pm) and C16–N2 (128.5 pm) are double C N bond. The C8 N1 and C16 N2 bonds are in the same plane with the corresponding pyridine rings, where deviations from the ring plane are roughly 1◦ . As expected [23–30], the carbon atoms of 1,4-butanediamine bridge are not in the same plane (Fig. 1). The C–C bond lengths in this region (15.7–154.8 pm) are appropriate values to a single bond. The calculated C–C–C, N–C–C and C–N–C angles are about 117◦ , 112◦ and 120◦ , respectively. The calculated N1· · ·H1 and N2· · ·H2 distances are 167.3 and 165.4 pm, respectively, indicating the intramolecular hydrogen bonds formation. The N· · ·H hydrogen bonding decreases the electron density in the binding region of the O1–H1 and O2–H2 bonds. Hence, the phenolic O–H bonds are longer than the alcoholic ones. The O1–H1 and O2–H2 bond lengths are 100.2 and 100.6 pm, respectively, whereas the O3–H9 and O4–H17 bond lengths are 96.7 pm. In addition, the electron attraction property of the aromatic rings decreases the electron density in the region of phenolic O–H bonds, too. The phenolic O–H bonds are weaker than the alcoholic ones, so, the acidity of phenolic protons is more and the ability of phenolic oxygens for coordination to metals is greater than the alcoholic ones. The phenolic O–H bonds are essentially in the same plane with the pyridine rings, where the calculated C5–C1–O1–H1 and C13–C9–O2–H2 dihedral angles are 2.7◦ and −0.9◦ , respectively. The V-shaped H2 BS ligand is not planar. Its molecule is twisted, so that the –CH3 , –CH2 OH and phenolic –OH groups are in opposite orientation compared with the corresponding groups on the other pyridine ring. Rotation of pyridine rings around the C17–N1 and C20–N2 single bonds put roughly the two substituted pyridine rings in the same plane. This provides structural requirements for the complex formation. The deprotonated BS2− acts as a tetradentate ligand framework, which has an N, N, O− , O− binding mode, via the deprotonated phenolic oxygens and the azomethine nitrogens. In the structure of Cu(II) complex (Fig. 2), four coordinated atoms are roughly in the same plane. The calculated O2–N2–N1–O1 and O1–O2–N2–Cu dihedral angles are 13.9◦ and 3.8◦ respectively. Therefore, the O2, N2 and N1 atoms are essentially in the same plane with the Cu atom. But the O1 atom has greater deviation

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Fig. 2. Structure and B3LYP optimized geometry of the [Cu(BS)(H2 O)(CH3 OH)] complex together with its labeling.

from planarity, where the calculated N2–N1–O1–Cu dihedral angle is 9.3◦ . The 6-membered rings created with the Cu atom are roughly planar, where the deviation is about 4◦ . Similar to the H2 BS ligand, the pyridine rings and their substituted groups are roughly in the same plane. Also, the 1,4-butanediamine bridge locates behind the complex. During complexation, deprotonation of the phenolic oxygens results in a decrease of the C1–O1 and C9–O2 bond lengths, in comparison with the free H2 BS ligand, by 5.6 and 3.8 pm, respectively. The Cu–O1 and Cu–O2 bond lengths are 197.8 and 197.1 pm, respectively, which are shorter than the Cu–N1 (202.6 pm) and Cu–N2 (204.9 pm) bond lengths. The O1, O2, N1 and N2 atoms of the BS2− Schiff-base ligand form the equatorial plane of the octahedral Cu complex, where H2 O and CH3 OH ligands locate in the axial positions. The H2 O ligand locates behind the complex (Fig. 2), with 257.9 pm length for the Cu–O5 bond. The Cu–O6 bond of CH3 OH ligand is longer than the Cu–O5 bond, by 19.7 pm. So, the axial Cu–O bonds are much longer than the equatorial ones. The H2 O and CH3 OH ligands are not completely perpendicular to the equatorial plane of the complex. The O5–Cu–C6, O5–Cu–C1 and O1–Cu–C6 angles are 143.4◦ , 74.7◦ and 83.4◦ , respectively. In comparison to the H2 BS, the complex formation results in a considerable decrease in the O1–O2 and N1–N2 distances, from 568.0 and 342.8 pm for the H2 BS to 272.8 and 305.5 pm for the Cu complex. Coordination of the azomethine nitrogens (N1 and N2 atoms) to the Cu, results in the elongation of the C–N bonds of the BS2− . The C8–N1 and N2–C17 bond lengths increase from 145.6 and 128.6 pm for the H2 BS to 129.7 and 147.8 pm for the Cu(II)(BS) complex. The calculated parameters for H2 BS and its Cu(II) complex are well in agreement with the previously reported values for the similar salen ligands and complexes [23–36], especially the compounds involving the 1,4-butanediamine bridge [23–30]. 3.2. Chemistry Here, the new H2 BS Schiff-base ligand and its Cu(II) complex were characterized by the elemental and spectroscopic (IR, UV–vis, 1 H NMR and mass) analysis. 3.2.1. Mass spectroscopy In the mass spectra of the H2 BS ligand and Cu(II) complex, molecular ion peaks, m/z (M+ ) were observed at 385 and 497, respectively. Also, a peak at 384 of the H2 BS spectrum is assigned to losing of the second phenolic H atom. Some of decomposed fragments of the Cu complex with the corresponding weight losses were assigned in terms of the proposed formula, and the data are given in Table 1. The decomposition mass losses and the results of elemental analyses are in good agreement with the proposed formula for the H2 BS ligand and its Cu(II) complex.

Table 1 Selected mass spectroscopic data of the [Cu(BS)(H2 O)(CH3 OH)] complex. Observed mass

Reduced mass (%)

Lossed fragments

497 464 443 384

0.02 6.83 11.04 22.89

Mother ion CH3 OH CH3 OH, H2 O Cu, CH3 OH, H2 O

3.2.2. Electronic spectra Electronic absorption spectra of the H2 BS ligand were recorded in DMSO and H2 O solutions. In the aqueous medium, four bands were observed. The absorption bands at 218 and 251 nm of the ligand spectrum is assigned to the ␲–␲* transitions of aromatic ring. The absorption bands at 315 and 400 nm are attributed to the ␲–␲* and n–␲* transitions of azomethine, respectively [37–39]. In DMSO solution, two absorption bands were observed in the UV–vis spectrum of H2 BS. These bands were appeared at 258 and 339 nm, which are assigned to the ␲–␲* transitions of the aromatic ring and azomethine, respectively. Electronic absorption spectrum of the Cu complex was recorded in DMSO solution. By the complex formation, absorption peaks undergo a considerable red shift, which leads to appearance of the ␲–␲* transitions at 270 and 393 nm. The coordination of the H2 BS is confirmed by these red shifts. The d–␲* transition band of the complex, as a MLCT transition, overlaps with the ␲–␲* transitions of azomethine, which has been appeared at 393 nm of the complex spectrum [40]. 3.3. 3.3. 1 H NMR spectra The experimental and theoretical 1 H NMR chemical shifts (ı) of the H2 BS Schiff-base ligand are given in Table 2, where the atom positions are numbered as in Fig. 1. The theoretical results are well in agreement with the experimental values, which confirm the suitability of optimized geometry for the H2 BS. The only exception is the alcoholic protons (H9 and H17), where the calculated chemical shifts are significantly lower than the values obtained from the experiment. The experimental Table 2 Experimental and theoretical chemical shifts butanediamine) in 1 H NMR spectrum, ı (ppm).

of

N,N -dipyridoxyl

(1,4-

Atom position

Exp.

Theo.

Atom position

Exp.

Theo.

H1, H2 H10 H3, H11 H18 H8 H15 H16 H7 H19 H25 H20 H26

14.11 8.90 7.82 8.90 4.60 4.60 4.60 4.60 3.70 3.70 3.70 3.70

14.69 9.80 9.28 9.08 6.86 5.91 5.66 5.56 5.03 4.96 4.30 3.87

H12 H13 H5 H4 H14 H6 H21 H23 H22 H24 H17 H9

2.46 2.46 2.46 2.46 2.46 2.46 1.72 1.72 1.72 1.72 5.39 5.39

3.68 3.48 3.37 3.28 3.14 3.03 3.14 2.96 2.67 2.45 0.63 0.46

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Table 3 Selected experimental and calculated IR vibrational frequencies (cm−1 ) of the N,N -dipyridoxyl (1,4-butanediamine) ligand and its Cu(II) complex. Experimental frequencies Ligand

Cu(II) complex

1080 (w) 1216 (m)

699 754 (w) 1002 (w) 1026 (vs) 1089 (w) 1198 (m)

1259 (m)

1265 (m)

1295 (m)

1320 (m)

1405 (vs)

1415 (vs)

1628 (vs)

1542 (w) 1618 (vs)

2841 (m) 2929 (w)

2857 (m) 2925 (w)

3048 (w)

3046 (w)

3414 (w, br)

3403 (m, br)

3727 (w)

3722 (m)

748 (w) 1013 (m)

Calculated frequencies

Vibrational assignment

Ligand

Cu(II) complex

– – 580

445 470 565 603 621 715 1055 1026 1097 1221

– 851 1054 – 1083 1232 1256 1294 – 1347 1360 1374 1392 1398 1399 – 1657 1658 2926 2880–2926, 2935, 2945, 2966 2937 2968 2971–2981 2951 2958 2992 – 3048 3092 – – 3640 –

1270 1315 1333 1345 1348 1362 1393 1417 1441 1451 1618 1631 1646 2858 2895, 2920, 2930, 2943 2938 2953 2956, 2989 2957 – 2963 2990 3012 3040 3085 3574 3581 3639 3653

(Cu–N, Cu–O) ␦(O–H) methanol ␦op (ring) ␦wag (O–H) H2 O asym Cu–O Breathing ␦(CH) Me + (ph–C) (C–O) methanol asym (ph–C–O)alc (ph–C) + (C1–C2, N3–C3, C4–C5, C9–C10, N4–C11, C13–C12) ␦ip (C–H) aromatic (C–O) phenolic ␦(C–O–H)methanol [␦twi (CH2 ) + ␦ip (OH)]alc ␦wag (CH2 ) bridge [␦wag (CH2 )]alc ␦(CH) Me ␦ip (C8–H10, C16–H18) (C1–O1) + (ring) + (ph–C) Left (C9–O2) + (ring) + (ph–C) right ␦sci (OH) H2 O (C8–N1) (C16–N2) sym (CH)alc sym (CH) bridge sym (CH) Me asym (CH)alc asym (CH) bridge (C8 H10 ) + (C16 –H18 ) (O–H)phenolic asym (CH) bridge asym (CH) Me asym (CH) methanol (C6–H6) + (C14–H14) (C–H) aromatic (O–H) methanol sym(O–H) H2 O (O–H)alc asym(O–H) H2 O

Abbreviation: sci, scissoring; wag, wagging; twi, twisting; op, out-of-plane; ip, in-plane; bridge, N(CH2 )4 N bridge region of molecules; alc, substituted –CH2 OH groups; left, left side of the molecule; right, right side of the molecule; Me, substituted –CH3 groups; w, weak; m, medium; s, strong; vs, very strong.

data are from (CD3 )2 CO solutions but the calculations correspond to the isolated molecule. Obviously, the solvent molecules interact with the alcoholic proton. Also, these protons can engage in intermolecular hydrogen bonds. The phenolic protons (H1, H2) are involved in the relevant intramolecular hydrogen bond interaction (O–H· · ·N), lead to the appearance of a signal at 14.11 ppm [36,41]. 3.4. Vibrational spectroscopy The selected vibrational frequencies of the H2 BS ligand and its Cu(II) complex are listed in Table 3. The vibrational modes were analyzed by comparing the data obtained from the literature reports [42–47] with the results of DFT calculations. Theoretical analysis of the IR and NMR spectra is an important tool to ensure the suitability of the proposed structures and identification of compounds [36,48]. The intensive band in the 1660–1500 cm−1 region of the IR spectra of Schiff-base ligands and complexes, is of particular interest since its energy is diagnostic of the mode of coordination of the ligands [36,42–47]. The IR spectrum of the complex compared with that of the H2 BS ligand shows that the (C N) band at 1628 cm−1 is shifted to lower energy by 10 cm−1 , indicating coordination of the ligand to Cu(II) through the azomethine nitrogens (N1 and N2 atoms) [36,40,42,46]. These very strong bands were assigned to the symmetrical stretching modes of C8 N1 and C16 N2 bonds.

Also, a comparison between the IR spectra of the H2 BS ligand and Cu complex shows other evidence, which can be used for identification of the complex structure. By the complex formation, the stretching vibrations of phenolic C–O bonds are appeared at higher frequencies (16 cm−1 ) than those of the ligand (1259 cm−1 ), indicating that the strength of these bonds have been increased by the deprotonation of ligand and increasing of electron density in this region. In comparison with the IR spectrum of H2 BS ligand, some new bands are appeared in the IR spectrum of Cu complex, most important of which are discussed here. A new weak band at 698 cm−1 was assigned to the asymmetrical stretching vibrations of Cu–O bonds. The very strong band at 1026 cm−1 confirms the presence of the methanol ligand in the complex structure. This band was attributed to the C21–O6 stretching vibrations. Also, the some bending and stretching vibrations of the methanol are overlapped with the other vibrations (Table 3). The weak band at 1542 cm−1 was assigned to the scissoring vibration of the H2 O ligand of the Cu complex. The stretching vibrations of the aromatic rings are appeared at 1534 cm−1 of the ligand IR spectrum, which shifts to 1527 cm−1 in the IR spectrum of the complex. The very strong bands which are appeared at 1405 and 1415 cm−1 of the IR spectra of ligand and complex, respectively, can be attributed to some overlapped vibrations (Table 3).

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In the 3600–2000 cm−1 spectral region of the IR spectra, stretching vibrations of the O–H bonds give rise to medium bands. These bands appear overlapped in the same spectral region together with each other and with the C–H stretching modes which cause to band broading [45,46]. The deconvolution of this region is listed in Table 3. As expected, the stretching vibrations of the phenolic O–H bonds are appeared at lower frequencies in comparison with the alcoholic O–H ones. Also, the stretching vibrations of the aliphatic C–H bonds include lower energies than the aromatic C–H ones. 4. Conclusion The H2 BS ligand and [Cu(BS)(H2 O)(CH3 OH)] complex have been synthesized and characterized by elemental analysis, UV–vis, 1 H NMR and IR spectroscopies. The optimized geometries show that the H2 BS is not planar, with a 70◦ dihedral angle between two pyridine rings. The phenolic protons engage in the intramolecular hydrogen bond (–O–H· · ·N). The H2 BS acts as a dianionic tetradentate ligand in N, N, O− , O− manner, via the deprotonated phenolic oxygens and the azomethine nitrogens. By complexation, the four coordinating atoms of BS2− put roughly in the same plane to each other and with Cu atom. This is achieved by the rotation of the substituted pyridine rings around the C17–N1 and C16–N2 bonds. The four atoms of BS2− occupy equatorial positions of the octahedral complex, where the axial ones are occupied by H2 O and CH3 OH ligands. In both of the H2 BS and Cu(II) complex, substituted groups are in the same plane with the pyridine rings. The calculated parameters are in good agreement with the reported results for the similar compounds. Also, the DFT calculated IR frequencies and 1 H NMR chemical shifts are well in agreement with the experimental results, which confirm suitability of the proposed geometries for the ligand and its Cu complex. Acknowledgment We gratefully acknowledge financial support from the Ferdowsi University of Mashhad (grant no. P-673). References [1] [2] [3] [4] [5] [6]

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