Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 151 (2015) 667–672
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Two new chelidamate complexes with the 4-methoxypyridine: A combined theoretical and experimental study _ Ibrahim Uçar a, Hatice Vural b,⇑, Ebru Küçük a a b
Department of Physics, Faculty of Arts and Sciences, Ondokuzmayıs University, Kurupelit, 55139 Samsun, Turkey Department of Electrical and Electronics Engineering, Faculty of Technology, Amasya University, 05000 Amasya, Turkey
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
Two new chelidamate complexes,
[M(chel)(mhpOCH3)2H2O]2H2O [M = Ni(II) (1); Co(II) (2); chel: chelidamate or 4-hydroxypyridi ne-2,6-dicarboxylate; mhp: 4-methoxypyridine] were synthesized. The M(II) complexes were characterized by structural (X-ray diffraction) and spectroscopic (infrared and UV–Vis) techniques. The geometry of the compounds was optimized with DFT-B3LYP method using 6-31G (d) basis set. The available experimental results were compared with theoretical data. The UV–Vis spectral data in methanol solvent have been computed and compared to the experiment.
a r t i c l e
i n f o
Article history: Received 5 April 2015 Received in revised form 1 July 2015 Accepted 3 July 2015 Available online 6 July 2015 Keywords: Chelidamic acid X-ray diffraction FT-IR UV–Vis DFT NBO
a b s t r a c t Two new mixed chelidamate complexes, [M(chel)(mhpOCH3)2H2O]2H2O [M = Ni(II) (1); Co(II) (2); chel: chelidamate or 4-hydroxypyridine-2,6-dicarboxylate; mhp: 4-methoxypyridine], were prepared and characterized through a combination of X-ray diffraction method, FT-IR and UV–Vis spectroscopy. The central M(II) ion in complex (1) and (2) is coordinated by the mhp nitrogen atom, the chel nitrogen and oxygen atoms and aqua oxygen atoms, forming the distorted octahedral geometry. Intra and intermolecular hydrogen bonds and p–p stacking interactions appear to be effective in the stabilization of the crystal structures. Also, the fully optimized geometries and vibrational frequencies have been calculated using density functional theory (DFT)-B3LYP with 6-31G (d) basis set. The vibrational frequencies were interpreted by means of potential energy distribution (PED). The energetic behaviors of the complexes in methanol solvent were examined using by time-dependent DFT (TD-DFT) method by applying the polarizable continuum model (PCM). The molecular stability and charge delocalization were analyzed using natural bond orbital (NBO) analysis. Ó 2015 Elsevier B.V. All rights reserved.
⇑ Corresponding author. E-mail address: [email protected] (H. Vural). http://dx.doi.org/10.1016/j.saa.2015.07.023 1386-1425/Ó 2015 Elsevier B.V. All rights reserved.
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1. Introduction 4-Hydroxypyridine-2,6-dicarboxylic acid or chelidamic acid (H2chel), is commonly used organic chemistry, biochemistry, coordinate chemistry, medical chemistry and even in HIV investigation [1–7]. A great deal of interest has been devoted to syntheses of chelidamic acid and its derivatives due to their various coordination modes and the potential applications in luminescent probe, radical adsorption and ferromagnetic interaction [8,9]. The studies on the transition metal complexes containing chelidamic acid proceed slowly. Till now, to the best of our knowledge, the only known examples consisting of Cr, Ag, Fe, Mn, Cu, V and Zn [2,10]. Transition metal complexes with chelidamic acid are suitable as hemoglobin producer in rabbits [11] and have beneficial effects in normalizing hyperglycemia in diabetic rats [12,13]. Chelidamic acid possesses antibacterial activity against Escherichia coli [14,15]. Electronic structure methods have been increasingly used by spectroscopies for modeling molecular properties that include stability structure and vibrational frequencies. Of late years, density functional theory (DFT) has been extensively used in theoretical modeling. The advancement of ever better exchange–correlation functionals has made it possible to calculate the large number of molecular properties with accuracies comparable to those of traditional correlated ab initio methods. In order to obtain information regarding the complexes [M(chel)(mhpOCH3)2H2O]2H2O [M = Ni(II) (1); Co(II) (2)] structures several techniques such as X-ray diffraction, UV–Vis, and FT-IR spectroscopies were used. The experimental studies on the Ni(II) and Co(II) chelidamate complexes with 4-methoxypyridine (mhp) ligand have been accompanied by density functional theory calculations. Vibrational properties and Highest occupied molecular orbital (HOMO) – Lowest unoccupied molecular orbital (LUMO) energies of the complexes were studied at the DFT/B3LYP with 6-31G (d) basis set [16,17]. TD-DFT was used to calculate the electronic absorption parameters of the complexes in the solvent environment using polarizable continuum model (PCM) and the results thus obtained were compared with the UV absorption spectra. 2. Experimental and theoretical methods 2.1. Physical measurements Electronic absorption spectra were measured in the range of 900–200 nm using Unicam UV2 UV–Vis spectrometer for methanol solution of complexes. The infrared spectra of all complexes were collected in the range of 4000–400 cm1 using a Vertex 80v Bruker FTIR spectrometer using KBr disc. 2.2. Crystal structure determination The crystal data were recorded on an Agilent Technologies SuperNova [18], (Single source at offset and Eos CCD detector) diffractometer with SuperNova (Mo) X-ray source with mirror-monochromated MoKa radiation wavelength of 0.71073 Å, at 293 K. The CrysAlisPro [18] software program was used for data collection, cell refinement, and data reduction. The structures were solved by direct-methods using SHELXS-97 [19]. Full-matrix least-squares refinement on F2 was carried out using SHELXL-97 [19]. All hydrogen atoms attached to carbon atoms were positioned geometrically and refined by a riding model with Uiso 1.2 times that of attached atoms and remaining hydrogen atoms were located from the Fourier difference map. In compounds 1 and 2, disorders of the asymmetric carbon and oxygen atoms of methoxy group were observed. All the disordered atoms were refined over two sites, labeled A and B. DIAMOND 3.0
(demonstrated version) [20] program was used the preparation of the figures. 2.3. Synthesis Chelidamic acid (10 mmol) dissolved in methanol/water (30 ml, ca 1:1 v/v) was neutralized with NaOH (20 mmol) and the resulting solution added to a hot solution of MCl23H2O [M = Ni(II), Co(II)] (10 mmol) in water. The reaction mixture was stirred at 300 K for 1 h and then cooled to room temperature. The resulting suspensions were filtered. A solution of 4-methoxypyridine (2 mmol) in methanol (10 ml) was added dropwise with stirring to a suspension of MChel3H2O (2 mmol) in methanol/water (20 ml). The mixture was refluxed for 1 h and stored 2 weeks. The green and pink crystals of suitable for X-ray diffraction analysis were harvested. 2.4. DFT calculations All electronic structure calculations have been performed at DFT method with 6-31 G (d) basis set using GAUSSIAN 03 program [21]. GAUSSIAN VIEW 03 software has been used visualization purposes [22]. The molecular structures of the title compounds in the ground state were optimized using DFT with hybrid functional B3LYP at 6-31G (d) basis set. On the basis of the optimized ground state geometry, the absorption spectral properties were calculated by TD-DFT approach associated with the polarisable continuum model (PCM). A detailed interpretation of the theoretical vibrational spectra was done on the basis of the potential energy distribution (PED) using VEDA4 [23] program. The absence of any imaginary frequency confirms that the complexes correspond to a true minimum on the potential energy surfaces. 3. Results and discussion 3.1. Molecular structure of complexes The crystallographic data of the complexes are summarized in Table 1. Two metal complexes crystallize in monoclinic space group P21/c with similar unit cell parameters. The corresponding interatomic bond distances and angles of 1 and 2 are collected in Table 2. The molecular structure of the complexes is presented in Fig. 1. The compounds 1 and 2 contain a neutral [M(HChel)(mhpOCH3)2H2O] [M = Ni(II) and Co(II)] unit together with crystal water molecules. Each M(II) ion is six-coordinate and their coordination geometries are defined as distorted octahedral. The Ni(II)–N2 bond distance is significantly longer than Ni(II)– N1 length (see Table 2) due to the two carboxylate groups in the ortho position. These carboxylate groups enhance the basicity of chelidamate nitrogen atom. The equatorial plane is formed by two pyridine nitrogen atoms and two carboxyl oxygen atoms and axial positions are occupied by two water molecules (Fig. 1). The M(II)–chel bond lengths [M(II)–Nchel and M(II)–Ochel] are in good agreement with the corresponding distances reported for Co(Chel)3H2OH2O0.25MeCN [Co–N = 2.036 Å; Co–O = 2.214 Å, 2.163 Å][24], (dmpH)[Co(H2Chel)(Hchel)]3H2O [Co–N = 2.005, 2.018 Å; Co–O = 2.159, 2.228, 2.209, 2.170 Å] [25], (Ni(Chel)3H2OH2O0.25MeCN) [Ni–N = 1.964 Å; Ni–O = 2.192, 2.117 Å] [24], (GH)2[Ni(HChel)2]2H2O (G = guanidin) [Ni–N = 1.970, 1.966 Å; Ni–O = 2.108, 2.214, 2.147, 2.138 Å] [26] and (dmpH)[Ni(HChel)(H2Chel)]2.35H2O (dmp = 2,9-dimetil-1,10-phenantrolin) [Ni–N = 1.961, 1.953 Å; Ni–O = 2.188, 2.137, 2.166, 2.129 Å] [27]. As shown in Fig. 1, in complexes, the asymmetric carbon and oxygen atoms of methoxy group were observed. Common atoms have labels with A, while the alternative
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3.2. Optimized geometry
Table 1 Crystal data and structure refinement for compounds. Formula
NiC13H18N2O10 (1)
CoC13H18N2O10 (2)
Formula weight (g) Temperature (K) Wavelength (Mo Å) Crystal system Space group Unit cell dimensions a, b, c (Å)
385 293 0.71073 Monoclinic P21/c
385.2 293 0.71073 Monoclinic P21/c
11.5553(4) 20.0099(5) 7.3507(4) 90 95.478(4) 90 1691.87 (12) 4 1.51
11.5590(4) 20.0410(6) 7.4660(3) 90 95.463(4) 90 1718.69 (10) 4 1.49
1.189
1.040
0.2 0.25 0.3 3.3–30.6 14 6 h 6 16 11 6 k 6 28 10 6 l 6 10
0.3 0.3 0.07 3.99–32.29 17 6 h 6 16 28 6 k 6 28 11 6 l 6 4
5196 254
5592 247
Integration Full-matrix leastsquares on F2
Integration Full-matrix leastsquares on F2
1.076 R1 = 0.059 wR2 = 0.109 0.42 and 0.34
1.041 R1 = 0.056 wR2 = 0.100 0.38 and 0.36
a, b, c (°) Volume (Å3) Z Calculated density (Mg m3) l (nm1) F(0,0,0) Crystal size (mm) h ranges (°) _ Index ranges
Reflections collected Independent reflections Reflection observed [I > 2r(I)] Absorption correction Refinement method Data/ restrains/parameters Goodness-of-fit on F2 Final R indices [I > 2r(I)] Largest diff. Peak and hole (e Å3)
0
4.4679(12) and 3.4548 Å A, respectively. Table 2 Selected structural parameters by X-ray and theoretical calculations for complexes. Complex 1 0
1.971(17) 2.026(19) 2.178(14) 2.121(15) 2.094(16) 2.095(15)
DFT/ B3LYP
Bond length
1.845 1.893 2.073 2.133 2.100 2.090
Co1–N1 Co1–N2 Co1–O1 Co1–O2 Co1–O7 Co1–O8
Bond angles (o) N1–Ni1–N2 O1–Ni1–O2 O7–Ni1–O8 N1–Ni1–O1 N1–Ni1–O2 N2–Ni1–O1
The Uv–Vis spectra of the compounds in methanol solvent were recorded within 200–900 nm range. In order to understand electron transition between the energy levels, the first 20 singlet ? singlet spin allowed excited states are considered and excitation energies, absorption wavelengths (k), oscillator strengths were obtained by the TD-DFT/PCM method. The contributions of the transitions were designated with the help of SWizard program [28]. Table 3 summarizes the experimental and calculated information about the UV–Vis spectra. In the complex 1, HOMO orbitals are localized on the chel (%96) and dNi (%3). d atomic orbitals play an essential role in the HOMO1 (% 0), HOMO2 (%48), HOMO3 (%63) and HOMO5 (%38) MOs, while HOMO1 (%57), HOMO4 (%96), HOMO8 (%59) and HOMO-9 (%62) are mainly formed by the chel p-bonding MOs (see Fig. S2). The mhp ligand plays an important role only in the HOMO6 (%34) and HOMO7 (%39) MOs. The electronic transitions with longest wavelengths are happened from the HOMO3(b) ? LUMO+1(b) ve HOMO3(b) ? LUMO+3(b). The
Complex 2 X-ray
(Å A) Ni1–N1 Ni1–N2 Ni1–O1 Ni1–O2 Ni1–O7 Ni1–O8
Full geometry optimization of compounds in the gas phase obtained in term of DFT/B3LYP levels with the 6-31G (d) basis set. Table 2 shows some optimized bond distances and angles of complexes. A superposition of the molecular structures of 1 and 2 as established by DFT method and XRD shows an excellent agreement (Fig. 2). The experimental Ni1–O1, Ni1–O2, Ni1–N1 and Ni1– N2 bond lengths for Ni(II) complex were defined as 2.178(14), 2.121(15), 1.971(17), and 2.026(19) Å. In theoretical calculations, these bond lengths calculated as 2.073, 2.133, 1.845 and 1.893 Å for B3LYP/6-31G (d) level. Therefore, there is good agreement between XRD and DFT results. The structural discrepancies between the calculated and observed geometry can be analyzed quantitatively by Root Mean Square Error (RMSE) overlay. The RMSE values of 0.165 and 0.186 were obtained for compounds 1 and 2, respectively. According to these results, there is a good correlation between the experimental and calculated geometries. The orientation of the pyridine rings of the chel for 1 and 2 are defined by the torsion angles C2–C1–C7–O1 [176.2(3)°] and C4–C5–C6–O2 [177.6(3)°] for the XRD results. These torsion angles are predicted as 163.753° and 170.896° for the DFT/B3LYP level, respectively. According to XRD study, the dihedral angles between the pyridine rings of the complexes are 18.23° for 1 and 17.08° for 2, whereas these angles are predicted as 14.2° and 25.76° for B3LYP/6-31G (d) level.
3.3. Electronic absorption spectra
methoxy group positions are labeled with B. The site occupancies for C13A/O6A and C13B/O6B are 0.55 and 0.45 for 1 and 0.49 for 2, respectively. There are intra and intermolecular O–H O hydrogen bonds interactions in the complexes, involving water molecules, carboxylate oxygen atoms and hydroxyl group (see Table S1). As a shown in Fig. S1, there is also symmetry-related face to face p–p stacking interaction between the two pyridine rings of chel ligand [ring A/B: C1/C2/C3/C4/C5/N1]. The centroid to centroid and centroid-to-plane distances between the pyridine rings are
Bond length
669
Experimental
DFT/ B3LYP
2.029(17) 2.079(19) 2.170(14) 2.209(15) 2.126(16) 2.123(15)
1.848 1.854 1.928 2.014 2.266 2.485
178.71(8) 151.60(6) 172.29(6) 76.50(6) 75.29(6) 102.64(7)
174.63 163.72 157.05 83.01 81.21 94.55
0
(Å A)
Bond angles (o) 179.56(8) 154.98(6) 174.64(6) 76.89(6) 78.13(6) 103.44(7)
179.16 162.45 160.77 81.70 80.86 97.76
N1–Co1–N2 O1–Co1–O2 O7–Co1–O8 N1–Co1–O1 N1–Co1–O2 N2–Co1–O1
Fig. 1. A view of [M(Chel)(mhp)2H2O]2H2O showing the atom-labeling scheme (30% probability ellipsoids) and hydrogen atoms are shown as small spheres of arbitrary radii.
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Comparison of calculated and experimental bands showed good agreement. 3.4. Vibrational frequencies
Fig. 2. Atom-by-atom superimposition of the title compounds calculated (red) over the X-ray structure (black). (DFT/UB3LYP/6-31G (d)). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
experimental bands at 280 nm and 236 nm are assigned as the p ? p⁄ intraligand (IL) transitions of the chel and mhp ligands. In the complex 2, HOMO orbitals are localized on dCo (%81), chel (%15) and mhp (%3). dCo orbitals play an essential role in the HOMO (%81), HOMO1 (%75) and HOMO2 (%63) MOs, while HOMO3 (%96), HOMO4 (%97), HOMO5 (%90), HOMO6 (%81), HOMO8 (%69) and HOMO9 (%68) are mainly formed by the chel p-bonding MOs. The mhp ligand plays an important role only in the HOMO7(%58) and HOMO8 (%22) MOs. The electronic transitions with longest wavelengths are happened from the HOMO1(a) ? LUMO+3(a) and HOMO(a) ? LUMO+3(a). The band at 239 nm is assigned as the p ? p⁄ intraligand (IL) transitions of the chel and mhp ligands. TD-DFT calculations of studied complexes in methanol solvent are listed in Table 3. From this table, we see that the experimental wavelength at 792–450–280–236 nm for complex 1 are predicted at 783–467–281–277 nm and can easily be seen that they correspond to the experimental absorption ones. The experimental absorptions bands at 559–493–418–239 nm for complex 2 are calculated at 573–483–417–238 nm using the B3LYP-PCM.
The infrared spectra of the title complexes were recorded in the region between 400 and 4000 cm1 using KBr discs. Harmonic vibrational frequencies were calculated by using the DFT method with 6-31G (d) basis set. The computed frequencies at DFT/B3LYP level were scaled with 0.96 [29]. The experimental and calculated values were compared in Table 4. The FT-IR spectra of the complexes show a broad band in the 3000–3500 cm1 region due to stretching vibrations of the O–H groups of aqua and chel ligands. The DFT calculations reproduce O–H stretching vibrations around 3593–3318 cm1 (1) and 3592–3321 cm1 (2). The experimental symmetric and asymmetric C–H stretching modes for the chel ligands were observed at 2963–2990 cm1 (1) and 2942–2990 cm1 (2). These vibrations were computed at 3126–3128 cm1 (1) and 3121, 3125 cm1 (2). The bands at 2505, 2628 cm1 (1) and 2497, 2622 cm1 (2) correspond to the symmetric and asymmetric C–H3 stretching vibrations of the mhp ligands and theoretically these bands has been calculated at 2932, 2997 cm1 and 2929, 2992 cm1 for 1 and 2, respectively. It is observed that the wavenumbers are over 2900 cm1, the calculated wavenumbers have some blue shifts compared to the experimental ones. This discrepancy can be explained by the fact that the intermolecular hydrogen bonding interaction with the neighboring molecules is absent in gas phase. The m(C@O) and m(C–O) stretching vibrations in the free chelidamic acid were found at 1718 cm1 and 1338 cm1, respectively [30]. The appeared bands at around 1617 cm1 and 1320 cm1 are assigned to m(C@O) and m(C–O) stretching vibrations for the complexes (Fig. 3). The difference between m(C@O) and m(C–O) of chel ligands is at near 300 cm1 indicating the presence of monodentate-coordinated carboxylate group [31]. According to
Table 3 Calculated electronic transitions for complexes with TD-DFT method. DFT/B3LYP (6-31G) Complexes
Experimental k (nm)
k (nm)
Osc. strength
Major contributions
792 450
783 467 348 340 316 311 303 297 291 290 286 281 279 277
0.0004 0.0003 0.0008 0.0004 0.0003 0.0004 0.0004 0.0018 0.0025 0.0023 0.0004 0.0005 0.0002 0.0002
H3(b)?L+1(b)(+65%) H4(b)?L+3(b)(+109%) H(a)?L(a)(+98%) H9(a)?L(a)(+34%) H3(a)?L+2(a)(+53%) H2(b)?L+(b)(+60%) H4(a)?L+1(a)(+24%) H4(b)?L(b)(+37%) H9(a)?L(a)(33%) H(a)?L+2(a)(+100%) H3(b)?L(b)(+100%) H3(b)?L(b)(+50%) H4(b)?L(b)(+72%) H3(b)?L(b)(+41%)
H3(b)?L+3(b)(38%) H3(b)?L+3(b)(32%) H(a)?L+1(a)(83%) H10(b)?L(b)(28%) H9(b)?L+2(b)(37%) H4(a)?L(a)(+38%) H5(b)?L(b)(23%) H5(a)?L+1(a)(26%) H10(b)?L(b)(+33%)
676 573 535 483 440 428 417 372 360 358 342 296 282 261 238
0.0005 0.0007 0.0007 0.0028 0.0096 0.0038 0.0034 0.0032 0.0089 0.0158 0.1374 0.0291 0.0222 0.0149 0.0021
H1(a)?L+3(a)(+130%) H3(a)?L+3(a)(+74%) H3(b)?L+3(b)(+96%) H2(b)?L+4b)(+90%) H2(b)?L(b)(+59%) H1(b)?L(b)(+93%) H2(b)?L(b)(+120%) H3(b)?L(b)(+55%) H3(b)?L(b)(+89%) H1(a)?L(a)(+65%) H2(a)?L+1(a)(+99%) H3(a)?L+1(a)(+35%) H2(b)?L+2(b)(+92%) H3(a)?L+2(a)(+83%) H6(b)?L+1(b)(+95%)
H(a)?L+3(a)(42%) H3(b)?L+4(b)(53%) H1(a)?L+3(a)(41%) H2(b)?L+3(b)(8%) H1(a)?L(a)(30%) H1(b)?L(b)(87%) H2(b)?L+1(b)(6%) H3(b)?L(b)(+40%) H2(b)?L(b)(+44%) H2(a)?L+1(a)(44%) H2(a)?L+1(a)(+5%) H2(a)?L+1(a)(21%) H3(b)?L+2(b)(13%) H5(a)?L+2(a)(+17%) H4(b)?L+1(b)(+5%)
Complex 1
280 236 559 493 Complex 2 418
239
H2(b)?L(b)(32%) H2(a)?L(a)(42%) H4(b)?L(b)(31%) H5(b)?L(b)(39%)
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Table 4 Comparison of the observed and calculated vibrational spectra of the complexes. PED(P10%)a assignments
Complex 1 1
m(OH2)aqua (50) m(OH)(chel)(50) mas(CH)(chel)(30) m(CH)(chel)(30) ms(CH)(mhp)(30) ms(CH)(mhp)(82) mas(CH3)(mhp)(48) ms(CH3)(mhp)(90) ms(C@O)(chel)(77) mas(C@O)(chel)(95) m(ring)(mhp)(88) m(ring)(chel)(40) [c(OH)(29)+ m(ring)(58)](chel) [c(CH)(29)+ m(ring)(57)](mhp) [c(CH)(29)+ m(ring)(50)](chel) m(C–O)(chel)(50) c(CH)(47)(chel) c(CH)(56)(mhp) Ring breathing (chel)(82) Ring breathing (mhp)(69) x(CH)(o.p)(chel)(79) x(CH)(o.p)(mhp)(73) c(ring)(chel)(72) c(ring)(mhp)(41) a b
Complex 2 b
Exp. cm
DFT
3536 3273 2990 2963 2852 2728 2628 2505 1617 1584 1570 1517 1458 1443 1415 1320 1169 1119 1055 1017 885 844 642 451
3593 3318 3128 3126 3115 3057 2997 2932 1679 1622 1613 1604 1571 1555 1443 1380 1087 1044 1033 999 874 834 636 469
Exp. cm
1
3540 3268 2990 2942 2852 2726 2622 2497 1617 1581 1569 1514 1457 1442 1415 1320 1166 1118 1061 1015 885 844 643 451
[Zn(HCAM)]nH2O [37]
(aacrH)2[Ni(hypydc)2]4H2O [38]
DFT 3592 3321 3125 3121 3132 3110 2992 2929 1662 1635 1615 1610 1572 1554 1443 1381 1089 1048 1046 1005 886 828 622 458
3598 3078
1638
1665
1469
1066 928 753
743
as: asymmetric, s: symmetric, m: stretching, c: in plane bending, x: wagging, o.p.: out of plane bending, chel: chelidamate. DFT/B3LYP method was applied.
Table 4, the calculated stretching vibration peak of the carboxylate groups appear at 1679 cm1 (1) and 1662 cm1 (2) while the peaks appear at 1617 cm1. These results indicate that the calculated vibrational frequencies are compatible to experimental values. The pyridine ring breathing vibrations in the compound appear at higher values compared to pyridine–metal complexes (995 cm1) [32,33]. It is known that there must be a considerable shift in the ring breathing vibrations of the structures containing pyridine nitrogen atom. The bands at 1055, 1061 cm1 (1) and 1017, 1015 cm1 (2) correspond to ring breathing mode of chel and mhp ligands, indicating that these ligands are attached to M(II) ions. These vibrations were calculated at 1033, 1046 cm1 (1) and 999, 1005 cm1 (2). The C–H in-plane bending wavenumbers appear in the range of 1000–1500 cm1 and C–H out-of-plane bending vibrations in the range of 700–1000 cm1. The in-plane C–H bending vibrations of the chel and mhp ligands were observed at 1169, 1119 cm1 for 1, 1166, 1118 cm1 for 2 experimentally, and calculated using B3LYP methods at 1087, 1044 cm1 for 1 and 1089, 1048 cm1 for 2, respectively. The out-of-plane C–H bending vibrations of the chel and mhp ligands were found at 885 and 844 cm1 for complexes experimentally, calculated at 874, 834 cm1 for 1 and 886, 828 cm1 for 2, respectively. It can be said that DFT-calculated frequencies are in good agreement than experimental values.
3.5. Natural bond orbital analysis The natural bond orbital (NBO) calculation of the compounds were performed using NBO 3.1 program implemented in the GAUSSIAN03 package at the DFT/B3LYP/6-31G (d) level. According to the NBO results, the electron numbers of complexes 3d, 4s and 4p orbits are 8.15, 0.30, 0.39 and 7.32, 0.28, 0.39 for 1 and 2, respectively (see Table S2). The electron numbers of 5p are smaller that can be neglected. Therefore, we can conclude that the M(II) atoms coordination with N1, N2, O1, O2, O7 and O8 atoms are on 3d, 4s and 4p orbits. The electron number of nitrogen atoms 2s orbit is 1.31, 2p that of orbit is 4.22 indicating that the N atoms
Fig. 3. Experimental and calculated FT-IR spectra of the title complexes.
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from coordination bonds with metal atoms by using 2s and 2p orbits. The coordinated O1, O2, O7 and O8 atoms provide electron of 2s (1.71, 1.69, 1.73, 1.72) and 2p (5.02, 5.10, 5.08, 5.23) to nickel and form the coordination bonds. The calculated natural charge on the metal atoms is equal to +1.14 (1) and 1.01 (2) deviating from the formal charge +2. This indicates a charge transfer from the ligands to the M(II) ion. The largest negative charges are located on the oxygen atoms O9 (0.99), O8 (0.96) and O7 (0.93) for Ni(II) complex and O9 (0.98), O7-O10 (-0.94) for Co (II) complex. The NBO analysis is a convenient tool for understanding delocalization of electron density from filled Lewis-type (donor) NBOs to properly empty non-Lewis type (acceptor) NBOs, which corresponds to a stabilizing donor–acceptor interaction [34]. The strength of this interaction can be predicted by the second-order perturbation theory. The second-order interaction energy E(2) value is calculated as [35,36],
Eð2Þ ¼ qi
ðF ij Þ2 ej ei
ð1Þ
where qi is the donor orbital occupancy, ei, ej are diagonal elements (orbital energies) and Fij is the off-diagonal NBO Fock matrix element. Decreasing of the electron occupancy in lonepair (LP) orbitals is the strongest evidence in delocalization. The second-order interaction energies have shown that the lone pairs localized on the nitrogen and oxygen atoms in compounds. The (E2) values of N1 atoms are 22.47 kcal/mol for complex 1 and 26.81 kcal/mol for complex 2. In addition, the second-order energy values of the chel O2 atoms are equal to 16.99 kcal/mol (1) and 15.92 kcal/mol (2). 4. Conclusions In this study, two chelidamate complexes were synthesized and characterized by structural (single crystal X-ray diffraction) and spectroscopic (FT-IR and UV–Vis) techniques. The geometries of the complexes have been optimized with DFT/B3LYP/6-31G (d) method. The results of structural investigation show that these compounds have six-coordinate geometry about the metal(II) centers. The crystal structures of the complexes from the XRD are compared with its optimized counterparts, indicating good agreement. The infrared spectra of the compounds were recorded and assigned with the help of the observed and calculated vibrational wavenumbers and their PED. The spectral data in methanol solvent were computed and compared to the experiment, indicating that TD-DFT/PCM calculations provide excellent results. The Metal– ligand interaction occurring instead by a delocalization from bond orbital (LP(1) N1) toward one antibonding orbital (LP*(5) Co and LP*(6) Ni), as estimated by second-order perturbation theory. Appendix A. Supplementary data Crystallographic data (excluding structure factors) for the structure in this paper have been deposited with the Cambridge Crystallographic Data Centre as the supplementary publication no CCDC 1055361 and 1055518 for 1 and 2, respectively. Copies of the data can be obtained, free of charge, on application to CCDC, 12 Union Road, Cambridge, CB12 1EZ, UK, fax: +44 1223 366 033, e-mail: [email protected] or on the web: http:// www.ccdc.cam.ac.uk. Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10. 1016/j.saa.2015.07.023.
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Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Two new chelidamate complexes with the 4-methoxypyridine: A combined theoretical and experimental study _ Ibrahim Uçar a, Hatice Vural b,⇑, Ebru Küçük a a b
Department of Physics, Faculty of Arts and Sciences, Ondokuzmayıs University, Kurupelit, 55139 Samsun, Turkey Department of Electrical and Electronics Engineering, Faculty of Technology, Amasya University, 05000 Amasya, Turkey
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
Two new chelidamate complexes,
[M(chel)(mhpOCH3)2H2O]2H2O [M = Ni(II) (1); Co(II) (2); chel: chelidamate or 4-hydroxypyridi ne-2,6-dicarboxylate; mhp: 4-methoxypyridine] were synthesized. The M(II) complexes were characterized by structural (X-ray diffraction) and spectroscopic (infrared and UV–Vis) techniques. The geometry of the compounds was optimized with DFT-B3LYP method using 6-31G (d) basis set. The available experimental results were compared with theoretical data. The UV–Vis spectral data in methanol solvent have been computed and compared to the experiment.
a r t i c l e
i n f o
Article history: Received 5 April 2015 Received in revised form 1 July 2015 Accepted 3 July 2015 Available online 6 July 2015 Keywords: Chelidamic acid X-ray diffraction FT-IR UV–Vis DFT NBO
a b s t r a c t Two new mixed chelidamate complexes, [M(chel)(mhpOCH3)2H2O]2H2O [M = Ni(II) (1); Co(II) (2); chel: chelidamate or 4-hydroxypyridine-2,6-dicarboxylate; mhp: 4-methoxypyridine], were prepared and characterized through a combination of X-ray diffraction method, FT-IR and UV–Vis spectroscopy. The central M(II) ion in complex (1) and (2) is coordinated by the mhp nitrogen atom, the chel nitrogen and oxygen atoms and aqua oxygen atoms, forming the distorted octahedral geometry. Intra and intermolecular hydrogen bonds and p–p stacking interactions appear to be effective in the stabilization of the crystal structures. Also, the fully optimized geometries and vibrational frequencies have been calculated using density functional theory (DFT)-B3LYP with 6-31G (d) basis set. The vibrational frequencies were interpreted by means of potential energy distribution (PED). The energetic behaviors of the complexes in methanol solvent were examined using by time-dependent DFT (TD-DFT) method by applying the polarizable continuum model (PCM). The molecular stability and charge delocalization were analyzed using natural bond orbital (NBO) analysis. Ó 2015 Elsevier B.V. All rights reserved.
⇑ Corresponding author. E-mail address: [email protected] (H. Vural). http://dx.doi.org/10.1016/j.saa.2015.07.023 1386-1425/Ó 2015 Elsevier B.V. All rights reserved.
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1. Introduction 4-Hydroxypyridine-2,6-dicarboxylic acid or chelidamic acid (H2chel), is commonly used organic chemistry, biochemistry, coordinate chemistry, medical chemistry and even in HIV investigation [1–7]. A great deal of interest has been devoted to syntheses of chelidamic acid and its derivatives due to their various coordination modes and the potential applications in luminescent probe, radical adsorption and ferromagnetic interaction [8,9]. The studies on the transition metal complexes containing chelidamic acid proceed slowly. Till now, to the best of our knowledge, the only known examples consisting of Cr, Ag, Fe, Mn, Cu, V and Zn [2,10]. Transition metal complexes with chelidamic acid are suitable as hemoglobin producer in rabbits [11] and have beneficial effects in normalizing hyperglycemia in diabetic rats [12,13]. Chelidamic acid possesses antibacterial activity against Escherichia coli [14,15]. Electronic structure methods have been increasingly used by spectroscopies for modeling molecular properties that include stability structure and vibrational frequencies. Of late years, density functional theory (DFT) has been extensively used in theoretical modeling. The advancement of ever better exchange–correlation functionals has made it possible to calculate the large number of molecular properties with accuracies comparable to those of traditional correlated ab initio methods. In order to obtain information regarding the complexes [M(chel)(mhpOCH3)2H2O]2H2O [M = Ni(II) (1); Co(II) (2)] structures several techniques such as X-ray diffraction, UV–Vis, and FT-IR spectroscopies were used. The experimental studies on the Ni(II) and Co(II) chelidamate complexes with 4-methoxypyridine (mhp) ligand have been accompanied by density functional theory calculations. Vibrational properties and Highest occupied molecular orbital (HOMO) – Lowest unoccupied molecular orbital (LUMO) energies of the complexes were studied at the DFT/B3LYP with 6-31G (d) basis set [16,17]. TD-DFT was used to calculate the electronic absorption parameters of the complexes in the solvent environment using polarizable continuum model (PCM) and the results thus obtained were compared with the UV absorption spectra. 2. Experimental and theoretical methods 2.1. Physical measurements Electronic absorption spectra were measured in the range of 900–200 nm using Unicam UV2 UV–Vis spectrometer for methanol solution of complexes. The infrared spectra of all complexes were collected in the range of 4000–400 cm1 using a Vertex 80v Bruker FTIR spectrometer using KBr disc. 2.2. Crystal structure determination The crystal data were recorded on an Agilent Technologies SuperNova [18], (Single source at offset and Eos CCD detector) diffractometer with SuperNova (Mo) X-ray source with mirror-monochromated MoKa radiation wavelength of 0.71073 Å, at 293 K. The CrysAlisPro [18] software program was used for data collection, cell refinement, and data reduction. The structures were solved by direct-methods using SHELXS-97 [19]. Full-matrix least-squares refinement on F2 was carried out using SHELXL-97 [19]. All hydrogen atoms attached to carbon atoms were positioned geometrically and refined by a riding model with Uiso 1.2 times that of attached atoms and remaining hydrogen atoms were located from the Fourier difference map. In compounds 1 and 2, disorders of the asymmetric carbon and oxygen atoms of methoxy group were observed. All the disordered atoms were refined over two sites, labeled A and B. DIAMOND 3.0
(demonstrated version) [20] program was used the preparation of the figures. 2.3. Synthesis Chelidamic acid (10 mmol) dissolved in methanol/water (30 ml, ca 1:1 v/v) was neutralized with NaOH (20 mmol) and the resulting solution added to a hot solution of MCl23H2O [M = Ni(II), Co(II)] (10 mmol) in water. The reaction mixture was stirred at 300 K for 1 h and then cooled to room temperature. The resulting suspensions were filtered. A solution of 4-methoxypyridine (2 mmol) in methanol (10 ml) was added dropwise with stirring to a suspension of MChel3H2O (2 mmol) in methanol/water (20 ml). The mixture was refluxed for 1 h and stored 2 weeks. The green and pink crystals of suitable for X-ray diffraction analysis were harvested. 2.4. DFT calculations All electronic structure calculations have been performed at DFT method with 6-31 G (d) basis set using GAUSSIAN 03 program [21]. GAUSSIAN VIEW 03 software has been used visualization purposes [22]. The molecular structures of the title compounds in the ground state were optimized using DFT with hybrid functional B3LYP at 6-31G (d) basis set. On the basis of the optimized ground state geometry, the absorption spectral properties were calculated by TD-DFT approach associated with the polarisable continuum model (PCM). A detailed interpretation of the theoretical vibrational spectra was done on the basis of the potential energy distribution (PED) using VEDA4 [23] program. The absence of any imaginary frequency confirms that the complexes correspond to a true minimum on the potential energy surfaces. 3. Results and discussion 3.1. Molecular structure of complexes The crystallographic data of the complexes are summarized in Table 1. Two metal complexes crystallize in monoclinic space group P21/c with similar unit cell parameters. The corresponding interatomic bond distances and angles of 1 and 2 are collected in Table 2. The molecular structure of the complexes is presented in Fig. 1. The compounds 1 and 2 contain a neutral [M(HChel)(mhpOCH3)2H2O] [M = Ni(II) and Co(II)] unit together with crystal water molecules. Each M(II) ion is six-coordinate and their coordination geometries are defined as distorted octahedral. The Ni(II)–N2 bond distance is significantly longer than Ni(II)– N1 length (see Table 2) due to the two carboxylate groups in the ortho position. These carboxylate groups enhance the basicity of chelidamate nitrogen atom. The equatorial plane is formed by two pyridine nitrogen atoms and two carboxyl oxygen atoms and axial positions are occupied by two water molecules (Fig. 1). The M(II)–chel bond lengths [M(II)–Nchel and M(II)–Ochel] are in good agreement with the corresponding distances reported for Co(Chel)3H2OH2O0.25MeCN [Co–N = 2.036 Å; Co–O = 2.214 Å, 2.163 Å][24], (dmpH)[Co(H2Chel)(Hchel)]3H2O [Co–N = 2.005, 2.018 Å; Co–O = 2.159, 2.228, 2.209, 2.170 Å] [25], (Ni(Chel)3H2OH2O0.25MeCN) [Ni–N = 1.964 Å; Ni–O = 2.192, 2.117 Å] [24], (GH)2[Ni(HChel)2]2H2O (G = guanidin) [Ni–N = 1.970, 1.966 Å; Ni–O = 2.108, 2.214, 2.147, 2.138 Å] [26] and (dmpH)[Ni(HChel)(H2Chel)]2.35H2O (dmp = 2,9-dimetil-1,10-phenantrolin) [Ni–N = 1.961, 1.953 Å; Ni–O = 2.188, 2.137, 2.166, 2.129 Å] [27]. As shown in Fig. 1, in complexes, the asymmetric carbon and oxygen atoms of methoxy group were observed. Common atoms have labels with A, while the alternative
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3.2. Optimized geometry
Table 1 Crystal data and structure refinement for compounds. Formula
NiC13H18N2O10 (1)
CoC13H18N2O10 (2)
Formula weight (g) Temperature (K) Wavelength (Mo Å) Crystal system Space group Unit cell dimensions a, b, c (Å)
385 293 0.71073 Monoclinic P21/c
385.2 293 0.71073 Monoclinic P21/c
11.5553(4) 20.0099(5) 7.3507(4) 90 95.478(4) 90 1691.87 (12) 4 1.51
11.5590(4) 20.0410(6) 7.4660(3) 90 95.463(4) 90 1718.69 (10) 4 1.49
1.189
1.040
0.2 0.25 0.3 3.3–30.6 14 6 h 6 16 11 6 k 6 28 10 6 l 6 10
0.3 0.3 0.07 3.99–32.29 17 6 h 6 16 28 6 k 6 28 11 6 l 6 4
5196 254
5592 247
Integration Full-matrix leastsquares on F2
Integration Full-matrix leastsquares on F2
1.076 R1 = 0.059 wR2 = 0.109 0.42 and 0.34
1.041 R1 = 0.056 wR2 = 0.100 0.38 and 0.36
a, b, c (°) Volume (Å3) Z Calculated density (Mg m3) l (nm1) F(0,0,0) Crystal size (mm) h ranges (°) _ Index ranges
Reflections collected Independent reflections Reflection observed [I > 2r(I)] Absorption correction Refinement method Data/ restrains/parameters Goodness-of-fit on F2 Final R indices [I > 2r(I)] Largest diff. Peak and hole (e Å3)
0
4.4679(12) and 3.4548 Å A, respectively. Table 2 Selected structural parameters by X-ray and theoretical calculations for complexes. Complex 1 0
1.971(17) 2.026(19) 2.178(14) 2.121(15) 2.094(16) 2.095(15)
DFT/ B3LYP
Bond length
1.845 1.893 2.073 2.133 2.100 2.090
Co1–N1 Co1–N2 Co1–O1 Co1–O2 Co1–O7 Co1–O8
Bond angles (o) N1–Ni1–N2 O1–Ni1–O2 O7–Ni1–O8 N1–Ni1–O1 N1–Ni1–O2 N2–Ni1–O1
The Uv–Vis spectra of the compounds in methanol solvent were recorded within 200–900 nm range. In order to understand electron transition between the energy levels, the first 20 singlet ? singlet spin allowed excited states are considered and excitation energies, absorption wavelengths (k), oscillator strengths were obtained by the TD-DFT/PCM method. The contributions of the transitions were designated with the help of SWizard program [28]. Table 3 summarizes the experimental and calculated information about the UV–Vis spectra. In the complex 1, HOMO orbitals are localized on the chel (%96) and dNi (%3). d atomic orbitals play an essential role in the HOMO1 (% 0), HOMO2 (%48), HOMO3 (%63) and HOMO5 (%38) MOs, while HOMO1 (%57), HOMO4 (%96), HOMO8 (%59) and HOMO-9 (%62) are mainly formed by the chel p-bonding MOs (see Fig. S2). The mhp ligand plays an important role only in the HOMO6 (%34) and HOMO7 (%39) MOs. The electronic transitions with longest wavelengths are happened from the HOMO3(b) ? LUMO+1(b) ve HOMO3(b) ? LUMO+3(b). The
Complex 2 X-ray
(Å A) Ni1–N1 Ni1–N2 Ni1–O1 Ni1–O2 Ni1–O7 Ni1–O8
Full geometry optimization of compounds in the gas phase obtained in term of DFT/B3LYP levels with the 6-31G (d) basis set. Table 2 shows some optimized bond distances and angles of complexes. A superposition of the molecular structures of 1 and 2 as established by DFT method and XRD shows an excellent agreement (Fig. 2). The experimental Ni1–O1, Ni1–O2, Ni1–N1 and Ni1– N2 bond lengths for Ni(II) complex were defined as 2.178(14), 2.121(15), 1.971(17), and 2.026(19) Å. In theoretical calculations, these bond lengths calculated as 2.073, 2.133, 1.845 and 1.893 Å for B3LYP/6-31G (d) level. Therefore, there is good agreement between XRD and DFT results. The structural discrepancies between the calculated and observed geometry can be analyzed quantitatively by Root Mean Square Error (RMSE) overlay. The RMSE values of 0.165 and 0.186 were obtained for compounds 1 and 2, respectively. According to these results, there is a good correlation between the experimental and calculated geometries. The orientation of the pyridine rings of the chel for 1 and 2 are defined by the torsion angles C2–C1–C7–O1 [176.2(3)°] and C4–C5–C6–O2 [177.6(3)°] for the XRD results. These torsion angles are predicted as 163.753° and 170.896° for the DFT/B3LYP level, respectively. According to XRD study, the dihedral angles between the pyridine rings of the complexes are 18.23° for 1 and 17.08° for 2, whereas these angles are predicted as 14.2° and 25.76° for B3LYP/6-31G (d) level.
3.3. Electronic absorption spectra
methoxy group positions are labeled with B. The site occupancies for C13A/O6A and C13B/O6B are 0.55 and 0.45 for 1 and 0.49 for 2, respectively. There are intra and intermolecular O–H O hydrogen bonds interactions in the complexes, involving water molecules, carboxylate oxygen atoms and hydroxyl group (see Table S1). As a shown in Fig. S1, there is also symmetry-related face to face p–p stacking interaction between the two pyridine rings of chel ligand [ring A/B: C1/C2/C3/C4/C5/N1]. The centroid to centroid and centroid-to-plane distances between the pyridine rings are
Bond length
669
Experimental
DFT/ B3LYP
2.029(17) 2.079(19) 2.170(14) 2.209(15) 2.126(16) 2.123(15)
1.848 1.854 1.928 2.014 2.266 2.485
178.71(8) 151.60(6) 172.29(6) 76.50(6) 75.29(6) 102.64(7)
174.63 163.72 157.05 83.01 81.21 94.55
0
(Å A)
Bond angles (o) 179.56(8) 154.98(6) 174.64(6) 76.89(6) 78.13(6) 103.44(7)
179.16 162.45 160.77 81.70 80.86 97.76
N1–Co1–N2 O1–Co1–O2 O7–Co1–O8 N1–Co1–O1 N1–Co1–O2 N2–Co1–O1
Fig. 1. A view of [M(Chel)(mhp)2H2O]2H2O showing the atom-labeling scheme (30% probability ellipsoids) and hydrogen atoms are shown as small spheres of arbitrary radii.
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Comparison of calculated and experimental bands showed good agreement. 3.4. Vibrational frequencies
Fig. 2. Atom-by-atom superimposition of the title compounds calculated (red) over the X-ray structure (black). (DFT/UB3LYP/6-31G (d)). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
experimental bands at 280 nm and 236 nm are assigned as the p ? p⁄ intraligand (IL) transitions of the chel and mhp ligands. In the complex 2, HOMO orbitals are localized on dCo (%81), chel (%15) and mhp (%3). dCo orbitals play an essential role in the HOMO (%81), HOMO1 (%75) and HOMO2 (%63) MOs, while HOMO3 (%96), HOMO4 (%97), HOMO5 (%90), HOMO6 (%81), HOMO8 (%69) and HOMO9 (%68) are mainly formed by the chel p-bonding MOs. The mhp ligand plays an important role only in the HOMO7(%58) and HOMO8 (%22) MOs. The electronic transitions with longest wavelengths are happened from the HOMO1(a) ? LUMO+3(a) and HOMO(a) ? LUMO+3(a). The band at 239 nm is assigned as the p ? p⁄ intraligand (IL) transitions of the chel and mhp ligands. TD-DFT calculations of studied complexes in methanol solvent are listed in Table 3. From this table, we see that the experimental wavelength at 792–450–280–236 nm for complex 1 are predicted at 783–467–281–277 nm and can easily be seen that they correspond to the experimental absorption ones. The experimental absorptions bands at 559–493–418–239 nm for complex 2 are calculated at 573–483–417–238 nm using the B3LYP-PCM.
The infrared spectra of the title complexes were recorded in the region between 400 and 4000 cm1 using KBr discs. Harmonic vibrational frequencies were calculated by using the DFT method with 6-31G (d) basis set. The computed frequencies at DFT/B3LYP level were scaled with 0.96 [29]. The experimental and calculated values were compared in Table 4. The FT-IR spectra of the complexes show a broad band in the 3000–3500 cm1 region due to stretching vibrations of the O–H groups of aqua and chel ligands. The DFT calculations reproduce O–H stretching vibrations around 3593–3318 cm1 (1) and 3592–3321 cm1 (2). The experimental symmetric and asymmetric C–H stretching modes for the chel ligands were observed at 2963–2990 cm1 (1) and 2942–2990 cm1 (2). These vibrations were computed at 3126–3128 cm1 (1) and 3121, 3125 cm1 (2). The bands at 2505, 2628 cm1 (1) and 2497, 2622 cm1 (2) correspond to the symmetric and asymmetric C–H3 stretching vibrations of the mhp ligands and theoretically these bands has been calculated at 2932, 2997 cm1 and 2929, 2992 cm1 for 1 and 2, respectively. It is observed that the wavenumbers are over 2900 cm1, the calculated wavenumbers have some blue shifts compared to the experimental ones. This discrepancy can be explained by the fact that the intermolecular hydrogen bonding interaction with the neighboring molecules is absent in gas phase. The m(C@O) and m(C–O) stretching vibrations in the free chelidamic acid were found at 1718 cm1 and 1338 cm1, respectively [30]. The appeared bands at around 1617 cm1 and 1320 cm1 are assigned to m(C@O) and m(C–O) stretching vibrations for the complexes (Fig. 3). The difference between m(C@O) and m(C–O) of chel ligands is at near 300 cm1 indicating the presence of monodentate-coordinated carboxylate group [31]. According to
Table 3 Calculated electronic transitions for complexes with TD-DFT method. DFT/B3LYP (6-31G) Complexes
Experimental k (nm)
k (nm)
Osc. strength
Major contributions
792 450
783 467 348 340 316 311 303 297 291 290 286 281 279 277
0.0004 0.0003 0.0008 0.0004 0.0003 0.0004 0.0004 0.0018 0.0025 0.0023 0.0004 0.0005 0.0002 0.0002
H3(b)?L+1(b)(+65%) H4(b)?L+3(b)(+109%) H(a)?L(a)(+98%) H9(a)?L(a)(+34%) H3(a)?L+2(a)(+53%) H2(b)?L+(b)(+60%) H4(a)?L+1(a)(+24%) H4(b)?L(b)(+37%) H9(a)?L(a)(33%) H(a)?L+2(a)(+100%) H3(b)?L(b)(+100%) H3(b)?L(b)(+50%) H4(b)?L(b)(+72%) H3(b)?L(b)(+41%)
H3(b)?L+3(b)(38%) H3(b)?L+3(b)(32%) H(a)?L+1(a)(83%) H10(b)?L(b)(28%) H9(b)?L+2(b)(37%) H4(a)?L(a)(+38%) H5(b)?L(b)(23%) H5(a)?L+1(a)(26%) H10(b)?L(b)(+33%)
676 573 535 483 440 428 417 372 360 358 342 296 282 261 238
0.0005 0.0007 0.0007 0.0028 0.0096 0.0038 0.0034 0.0032 0.0089 0.0158 0.1374 0.0291 0.0222 0.0149 0.0021
H1(a)?L+3(a)(+130%) H3(a)?L+3(a)(+74%) H3(b)?L+3(b)(+96%) H2(b)?L+4b)(+90%) H2(b)?L(b)(+59%) H1(b)?L(b)(+93%) H2(b)?L(b)(+120%) H3(b)?L(b)(+55%) H3(b)?L(b)(+89%) H1(a)?L(a)(+65%) H2(a)?L+1(a)(+99%) H3(a)?L+1(a)(+35%) H2(b)?L+2(b)(+92%) H3(a)?L+2(a)(+83%) H6(b)?L+1(b)(+95%)
H(a)?L+3(a)(42%) H3(b)?L+4(b)(53%) H1(a)?L+3(a)(41%) H2(b)?L+3(b)(8%) H1(a)?L(a)(30%) H1(b)?L(b)(87%) H2(b)?L+1(b)(6%) H3(b)?L(b)(+40%) H2(b)?L(b)(+44%) H2(a)?L+1(a)(44%) H2(a)?L+1(a)(+5%) H2(a)?L+1(a)(21%) H3(b)?L+2(b)(13%) H5(a)?L+2(a)(+17%) H4(b)?L+1(b)(+5%)
Complex 1
280 236 559 493 Complex 2 418
239
H2(b)?L(b)(32%) H2(a)?L(a)(42%) H4(b)?L(b)(31%) H5(b)?L(b)(39%)
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Table 4 Comparison of the observed and calculated vibrational spectra of the complexes. PED(P10%)a assignments
Complex 1 1
m(OH2)aqua (50) m(OH)(chel)(50) mas(CH)(chel)(30) m(CH)(chel)(30) ms(CH)(mhp)(30) ms(CH)(mhp)(82) mas(CH3)(mhp)(48) ms(CH3)(mhp)(90) ms(C@O)(chel)(77) mas(C@O)(chel)(95) m(ring)(mhp)(88) m(ring)(chel)(40) [c(OH)(29)+ m(ring)(58)](chel) [c(CH)(29)+ m(ring)(57)](mhp) [c(CH)(29)+ m(ring)(50)](chel) m(C–O)(chel)(50) c(CH)(47)(chel) c(CH)(56)(mhp) Ring breathing (chel)(82) Ring breathing (mhp)(69) x(CH)(o.p)(chel)(79) x(CH)(o.p)(mhp)(73) c(ring)(chel)(72) c(ring)(mhp)(41) a b
Complex 2 b
Exp. cm
DFT
3536 3273 2990 2963 2852 2728 2628 2505 1617 1584 1570 1517 1458 1443 1415 1320 1169 1119 1055 1017 885 844 642 451
3593 3318 3128 3126 3115 3057 2997 2932 1679 1622 1613 1604 1571 1555 1443 1380 1087 1044 1033 999 874 834 636 469
Exp. cm
1
3540 3268 2990 2942 2852 2726 2622 2497 1617 1581 1569 1514 1457 1442 1415 1320 1166 1118 1061 1015 885 844 643 451
[Zn(HCAM)]nH2O [37]
(aacrH)2[Ni(hypydc)2]4H2O [38]
DFT 3592 3321 3125 3121 3132 3110 2992 2929 1662 1635 1615 1610 1572 1554 1443 1381 1089 1048 1046 1005 886 828 622 458
3598 3078
1638
1665
1469
1066 928 753
743
as: asymmetric, s: symmetric, m: stretching, c: in plane bending, x: wagging, o.p.: out of plane bending, chel: chelidamate. DFT/B3LYP method was applied.
Table 4, the calculated stretching vibration peak of the carboxylate groups appear at 1679 cm1 (1) and 1662 cm1 (2) while the peaks appear at 1617 cm1. These results indicate that the calculated vibrational frequencies are compatible to experimental values. The pyridine ring breathing vibrations in the compound appear at higher values compared to pyridine–metal complexes (995 cm1) [32,33]. It is known that there must be a considerable shift in the ring breathing vibrations of the structures containing pyridine nitrogen atom. The bands at 1055, 1061 cm1 (1) and 1017, 1015 cm1 (2) correspond to ring breathing mode of chel and mhp ligands, indicating that these ligands are attached to M(II) ions. These vibrations were calculated at 1033, 1046 cm1 (1) and 999, 1005 cm1 (2). The C–H in-plane bending wavenumbers appear in the range of 1000–1500 cm1 and C–H out-of-plane bending vibrations in the range of 700–1000 cm1. The in-plane C–H bending vibrations of the chel and mhp ligands were observed at 1169, 1119 cm1 for 1, 1166, 1118 cm1 for 2 experimentally, and calculated using B3LYP methods at 1087, 1044 cm1 for 1 and 1089, 1048 cm1 for 2, respectively. The out-of-plane C–H bending vibrations of the chel and mhp ligands were found at 885 and 844 cm1 for complexes experimentally, calculated at 874, 834 cm1 for 1 and 886, 828 cm1 for 2, respectively. It can be said that DFT-calculated frequencies are in good agreement than experimental values.
3.5. Natural bond orbital analysis The natural bond orbital (NBO) calculation of the compounds were performed using NBO 3.1 program implemented in the GAUSSIAN03 package at the DFT/B3LYP/6-31G (d) level. According to the NBO results, the electron numbers of complexes 3d, 4s and 4p orbits are 8.15, 0.30, 0.39 and 7.32, 0.28, 0.39 for 1 and 2, respectively (see Table S2). The electron numbers of 5p are smaller that can be neglected. Therefore, we can conclude that the M(II) atoms coordination with N1, N2, O1, O2, O7 and O8 atoms are on 3d, 4s and 4p orbits. The electron number of nitrogen atoms 2s orbit is 1.31, 2p that of orbit is 4.22 indicating that the N atoms
Fig. 3. Experimental and calculated FT-IR spectra of the title complexes.
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from coordination bonds with metal atoms by using 2s and 2p orbits. The coordinated O1, O2, O7 and O8 atoms provide electron of 2s (1.71, 1.69, 1.73, 1.72) and 2p (5.02, 5.10, 5.08, 5.23) to nickel and form the coordination bonds. The calculated natural charge on the metal atoms is equal to +1.14 (1) and 1.01 (2) deviating from the formal charge +2. This indicates a charge transfer from the ligands to the M(II) ion. The largest negative charges are located on the oxygen atoms O9 (0.99), O8 (0.96) and O7 (0.93) for Ni(II) complex and O9 (0.98), O7-O10 (-0.94) for Co (II) complex. The NBO analysis is a convenient tool for understanding delocalization of electron density from filled Lewis-type (donor) NBOs to properly empty non-Lewis type (acceptor) NBOs, which corresponds to a stabilizing donor–acceptor interaction [34]. The strength of this interaction can be predicted by the second-order perturbation theory. The second-order interaction energy E(2) value is calculated as [35,36],
Eð2Þ ¼ qi
ðF ij Þ2 ej ei
ð1Þ
where qi is the donor orbital occupancy, ei, ej are diagonal elements (orbital energies) and Fij is the off-diagonal NBO Fock matrix element. Decreasing of the electron occupancy in lonepair (LP) orbitals is the strongest evidence in delocalization. The second-order interaction energies have shown that the lone pairs localized on the nitrogen and oxygen atoms in compounds. The (E2) values of N1 atoms are 22.47 kcal/mol for complex 1 and 26.81 kcal/mol for complex 2. In addition, the second-order energy values of the chel O2 atoms are equal to 16.99 kcal/mol (1) and 15.92 kcal/mol (2). 4. Conclusions In this study, two chelidamate complexes were synthesized and characterized by structural (single crystal X-ray diffraction) and spectroscopic (FT-IR and UV–Vis) techniques. The geometries of the complexes have been optimized with DFT/B3LYP/6-31G (d) method. The results of structural investigation show that these compounds have six-coordinate geometry about the metal(II) centers. The crystal structures of the complexes from the XRD are compared with its optimized counterparts, indicating good agreement. The infrared spectra of the compounds were recorded and assigned with the help of the observed and calculated vibrational wavenumbers and their PED. The spectral data in methanol solvent were computed and compared to the experiment, indicating that TD-DFT/PCM calculations provide excellent results. The Metal– ligand interaction occurring instead by a delocalization from bond orbital (LP(1) N1) toward one antibonding orbital (LP*(5) Co and LP*(6) Ni), as estimated by second-order perturbation theory. Appendix A. Supplementary data Crystallographic data (excluding structure factors) for the structure in this paper have been deposited with the Cambridge Crystallographic Data Centre as the supplementary publication no CCDC 1055361 and 1055518 for 1 and 2, respectively. Copies of the data can be obtained, free of charge, on application to CCDC, 12 Union Road, Cambridge, CB12 1EZ, UK, fax: +44 1223 366 033, e-mail: [email protected] or on the web: http:// www.ccdc.cam.ac.uk. Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10. 1016/j.saa.2015.07.023.
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