Comparison between the calculated and experimental results covering molecular structures, assignment of fundamental vibrational modes, and thermodynamic properties were investigated. The optimized molecular geometries were compared with the experimental data obtained from X-ray data of a similar complex, which indicated that the theoretical results agree with the corresponding experimental values. The UV-Vis spectrum of the compound was also recorded and some properties, such as HOMO and LUMO energies and λmax , were determined using DFT (PW91) method. The absorption wavelengths were compared with the experimental data.
Turkish Journal of Chemistry http://journals.tubitak.gov.tr/chem/ Research Article Turk J Chem (2013) 37: 867 878 ă ITAK c TUB ⃝ doi:10.3906/kim-1207-74 Synthesis, characterization, and DFT calculation of a Pd(II) Schiff base complex Alireza AKBARI,∗ Zahra ALINIA Department of Chemistry, Payame Noor University, Tehran, Iran Received: 30.07.2012 • Accepted: 04.03.2013 • Published Online: 04.11.2013 • Printed: 29.11.2013 Abstract: The 4-((E)-(2-((E)-2, 4-dihydroxybenzylideneamino) ethylimino) methyl) benzene-1,3-diol tetradentate ligand, H L, reacted with PdCl to produce the related complex The complex was characterized by elemental analysis, infrared and electronic spectroscopy, thermogravimetric study, and molar conductance Furthermore, the fully optimized geometries were calculated using the ADF 2009.01 package Comparison between the calculated and experimental results covering molecular structures, assignment of fundamental vibrational modes, and thermodynamic properties were investigated The optimized molecular geometries were compared with the experimental data obtained from X-ray data of a similar complex, which indicated that the theoretical results agree with the corresponding experimental values The UV-Vis spectrum of the compound was also recorded and some properties, such as HOMO and LUMO energies and λmax , were determined using DFT (PW91) method The absorption wavelengths were compared with the experimental data Key words: Salen, Schiff bases, palladium(II), thermogravimetric, DFT Introduction The chemistry of Schiff bases has occupied a place of considerable importance because of their well-established biological properties They are well-known antibacterial as well as antifungal agents 1−4 Considerable importance has been given to transition metal complexes of these ligands on account of their biological properties 5−7 Various transition metal complexes with some Schiff bases containing nitrogen and oxygen donor atoms play an important role in biological systems and represent interesting models for metal enzymes, which efficiently catalyze the reduction of dinitrogen and dioxygen 8−10 Furthermore, the macrocyclic derivatives of these Schiff bases were found to have many fundamental biological functions such as photosynthesis 11,12 and transport of oxygen in mammalian 13 and other respiratory systems 14 The Schiff base complexes of transition metal ions can be applied, particularly in the development of the agrochemical and pharmaceutical industries 15 The metal centers of most reported complexes, [M (Salen)], belong to the first row of transition metals and less attention has been paid to complexes of metals of the second and third rows, such as palladium or platinum The coordination chemistry of the square planar palladium(II) and platinum(II) complexes of nitrogen and sulfur-oxygen donor ligands have gained importance because of their antitumor, anticancer, and catalytic activities 16−18 Antiandrogen and antimicrobial aspects of coordination compounds of palladium(II) and platinum(II) have also been reported in recent years 19−22 The antifertility activity of Schiff base complexes of palladium(II) and platinum(II) in male albino rats has also been reported 23,24 [Pd(salen)] complexes have been declared as efficient catalysts for hydrogenation of imines, 25,26 the Heck olefination of aryl iodides, and the Suzuki reaction of aryl ∗ Correspondence: a akbari@pnu.ac.ir 867 AKBARI and ALINIA/Turk J Chem iodides and bromides under aerobic conditions 27 However, publications devoted to the electronic and structural properties of these complexes are still rare Taking these facts into consideration, we report herein the synthesis and spectroscopic investigation of the palladium(II) complex with an N, O based ligand The complex was described by quantum-mechanical DFT calculations that were performed on it Molecular geometries, infrared frequencies, and electronic energy levels (for the frontier molecular orbitals, highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO)) were estimated and correlated, whenever possible, with experimental data Experimental 2.1 Synthesis and computational and physical methods All chemicals and solvents were obtained from Merck and Fluka They were used without further purification The infrared spectra of the ligand and complex were recorded on a Shimadzu Prestige 8400 FT-IR spectrometer as KBr disks Elemental CHN analyses were performed using a Heraeus CHN-O-FLASH EA 1112 elemental analyzer The conductivity measurements were carried out in DMSO at room temperature using a Metrohm 712 conductivity meter The electronic spectra were recorded in DMSO on a Shimadzu 2550 (UV-Vis) spectrophotometer Thermogravimetric analysis (TGA) was carried out using a TGA-50 SHIMADZU under air atmosphere over the temperature range of 20–800 ◦ C at a heating rate of 10 ◦ C/min The Schiff base ligand, H L, was prepared according to the procedure described in the literature 28 A solution of dihydroxy benzaldehyde (2 mmol in 20 mL of ethanol) and the related diamine (1 mmol in 20 mL of ethanol) were refluxed for h on a water bath The resulting suspension was kept at room temperature prior to being filtered, washed with ethanol, and dried with diethyl ether to afford the required Schiff base The product was recrystallized from ethanol for further purification Orange solid, yield 92%; m.p > 250 ◦ C dec.IR (KBr, cm −1 ), 3478, 2932, 2893, 2837, 2454, 1635, 602, 561; Anal calcd for H L (%): C, 63.99; H, 5.37; N, 9.33 Found: C, 62.90; H, 5.13; N, 9.45 UV-Vis (DMSO) λmax (nm) ( ε (M −1 cm −1 )): 280 (22,240), 308 (18360), 376 (2720) Conductivity (DMSO): 2.38 Ω−1 cm mol −1 The H L ligand (0.06 g, 0.2 mmol) and triethylamine (75 µ L) were dissolved in mL of ethanol and added dropwise to a suspension of PdCl (0.035 g, 0.2 mmol) in mL of ethanol The mixture was stirred for day at room temperature Filtering, washing by ethanol, and drying in air gave a green solid, yield: 60%; m.p 235 ◦ C dec.; IR (KBr, cm −1 ): 3455, 2898, 2854, 2831, 1612, 1539, 1451, 563, 471; Anal calcd for PdL.2H O (%): C, 43.60; H, 4.12; N, 6.36 Found: C, 42.68; H, 4.16; N, 5.88 UV-Vis (DMSO) λmax (nm) (ε (M −1 cm −1 )): 288 (21820), 297 (21,980), 365 (9780); Conductivity (DMSO): 8.81 Ω−1 cm mol −1 The quantum chemical calculations were performed with the Amsterdam density functional (ADF) 2009.01 package 29 The geometries of the ligand and complex were optimized at the PW91 level and DZ, DZP, and TZP basis sets 30,31 Calculations of harmonic frequencies at the fully optimized geometries of the ligand and its Pd(II) complex were performed at the PW91 level and TZP basis set Firstly, the geometry taken from the starting structure was fully optimized (Figure 1) Secondly, the UV-Vis spectral properties were explained and illustrated from the frontier molecular orbital orientation Thirdly, the harmonic frequencies were calculated to convince the optimized structures to be in their stable ground states Finally, the calculated vibrational spectra were assigned based on a vibrational mode analysis using the ADF program Two scale factors, 0.9648 and 0.72, were used for correction of the calculated harmonic vibrational frequencies and singlet–singlet calculated excitations, respectively 32,33 868 AKBARI and ALINIA/Turk J Chem Results and discussion The gas phase optimized structures of the ligand and Pd(II) complex with labeling of the atoms are presented in Figures and 2, respectively The optimized structural parameters, bond lengths, bond angles, and dihedral angles for the energetically and thermodynamically preferred structure of the PdL complex obtained from DFT calculations, PW91 method, are listed in Table along with the available X-ray data of similar compounds 34 C1 C C5 N4 C6 H30 H29 C 10 C C 11 N3 C 14 O8 C 19 C 24 O25 C 20 C 12 C 13 C 23 C 21 C 22 O17 O27 H 32 H 31 Figure Optimized structure for H L at the PW91/TZP level of theory together with its labeling C1 C5 N4 C 10 C C 11 C2 N3 Pd C 14 O8 C 12 C 13 O17 C6 C 19 C 24 O25 C 20 C 23 C 21 C 22 O27 H 32 H 31 Figure Optimized structure for Pd complex at the PW91/TZP level of theory with its atom labeling The H L molecule deprotonates and then acts as a tetradentate dianionic Schiff-base ligand framework, ˚) and C –N (1.295 ˚ which has a N, N, O − , O − binding mode The calculated C –N (1.324 A A) bond lengths ˚) and C –N (1.448 are of the approximate value for a C =N double bond length, whereas the N –C (1.446 A ˚ A) bond lengths are a bit shorter than a C–N single bond The calculated C 14 –O and C 20 –O 25 bond distances are 1.282 and 1.341 ˚ A, respectively, which are somewhat between the length of a C–O single bond (1.43 ˚ A) and that of a C= O double bond (1.20 ˚ A) These values suggest that electron density may be delocalized throughout the N –C –C 19 fragment, the aromatic ring, and the C 20 –O 25 bond on the right side of the ligand molecule, i.e the appearance of the π -electron resonance in this region Furthermore, on the left side of this molecule, π -electrons are delocalized throughout the N –C –C fragment, the aromatic ring, and the C 14 -O bond An intermolecular hydrogen bond, O –H 30 N is observed in the molecule 35 The O –H 30 bond length is 1.551 ˚ A This value is larger than O 25 –H 29 , O 17 –H 31 , and O 27 –H 32 ˚, respectively The calculated N H 30 distance is 1.083 ˚ bond lengths, which are 1.019, 0.974, and 0.973 A A, which demonstrates that there is a hydrogen bond between these atoms The internuclear distances determined between O 25 and H 29 , and N and H 29 in the optimized geometries are 1.019 and 1.677 ˚ A, respectively This longer distance is not in favor of the formation of an intermolecular hydrogen bond of the type O 25 –H 29 N The engagement in intermolecular hydrogen bond interactions causes the longer O –H 30 bond than O 25 –H 29, 869 AKBARI and ALINIA/Turk J Chem ˚) bond distance is shorter than C 20 –O 25 O 17 –H 31 , and O 27 –H 32 On the other hand, the C 14 –O (1.282 A ˚), C 22 –O 27 (1.372 ˚ (1.341 A A), and C 12 –O 17 (1.373 ˚ A) These values indicate that the electron density at the carbon–oxygen bond for C 14 –O is higher than for C 20 –O 25 , C 22 –O 27 , and C 12 –O 17 bonds Inversely, the electron density at the O –H 30 bond is lower than in O 25 –H 29 , O 17 –H 31 , and O 27 –H 32 bonds Therefore, the acidic nature of the H 30 proton of the O –H 30 bond is more than for O 25 –H 29 , O 17 –H 31 , and O 27 –H 32 Hence, the O atom has a higher ability for coordination to the metal ion than O 25 , O 17 , and O 27 atoms In the complexation region, deprotonating of the phenolic OH group causes a decrease in the C 20 –O 25 bond length, while coordination of the N atom to the Pd(II) ion leads to elongation of the C –N bond Table Some selected bond parameters of H L ligand and PdL complex H2L (calculated) Pd L (calculated) a DZ DZP TZP DZ DZP TZP a C5–N4 1.339 1.320 1.324 1.312 1.298 1.302 1.293 N4–C1 1.463 1.443 1.446 1.485 1.462 1.464 1.477 C2–N3 1.472 1.446 1.448 1.486 1.462 1.464 1.477 N3–C6 1.311 1.292 1.295 1.311 1.297 1.301 1.294 C14–O8 1.308 1.277 1.282 1.333 1.295 1.302 1.314 Pd–O8 - - - 2.095 2.080 2.078 1.998 Pd–O25 - - - 2.095 2.078 2.076 1.991 Pd–N4 - - - 2.049 2.023 2.025 1.954 Pd–N3 - - - 2.049 2.023 2.025 1.956 O8–Pd–N3 - - - 174.6 175.0 174.7 178.2 N4– Pd–O25 - - - 174.7 175.1 174.9 177.2 N4–Pd–N3 - - - 83.3 82.8 82.8 84.2 N3–Pd–O25 - - - 91.7 92.5 92.4 94.3 O25–Pd–O8 - - - 93.4 92.2 92.5 87.4 O 8–Pd–N4 - - - 91.7 92.5 92.3 94.2 X-ray Bond length (Å) Bond angle (°) a Data come from Ref 34 for similar palladium complex; N,N-ethane-1,2-diyl-bis(3-methyl-1-salicylideneiminate) palla- dium(II) The C= C bond distances of the aromatic rings in the ligand are in the expected range 36 The calculated dihedral angles, C 19 –C 24 –C 23 –H 28 (180 ◦ ) and C –C 10 –C 11 –H 16 (180 ◦ ) , indicate that aromatic hydrogens, H 28 and H 16 , are roughly in the same plane of the rings Due to the existing O –H 30 N hydrogen bond, the C –N bond length decreases after complex formation while the C 14 –O bond length increases The calculated dihedral angles C 24 –N 23 –C 22 –O 27 (179.8 ◦ ), C 10 –C 11 –C 12 –O 17 (179.9 ◦ ) , C 23 –C 22 –O 27 –H 32 (179.6 ◦ ), and C 11 –C 12 –O 17 –H 31 (180.1 ◦ ) show that the substituted phenolic groups along with their H 32 and H 31 atoms are roughly in the same plane with the rings The benzene rings are essentially planar and make an approximately 870 AKBARI and ALINIA/Turk J Chem 17 ◦ dihedral angle, C 12 –C –O 19 –C 22 , with each other Theoretical angle values for the Pd(II) complex show some deviation from the square-planer geometry For example, the calculated angles obtained for N –Pd–O 25 and O –Pd–N are 177.15 ◦ and 178.2 ◦ respectively, instead of 180 ◦ As can be seen from Table 1, the agreement between the experimental and theoretical results obtained at this level of calculation is reasonably good The observed differences are most probably because the calculations were performed in the gas phase, whereas the experimental results were obtained in the solid phase Atomic charges of the H L ligand and PdL complex calculated by the Mulliken method 37 at the PW91/TZP level of theory are given in Table As can be seen, higher charge density was found at C 14 than for the other ring carbon atoms on the ligand On the other hand, C 14 , which is connected to oxygen atom (O ), has the maximum charge (–0.6465) The very less negative charge of the oxygen atom (O ) is due to the presence of N–H· · · O intermolecular hydrogen bonding Table The Mulliken atomic charges of ligand and complex calculated with pw91/TZP method Ligand Complex Ligand Complex Ligand Complex C1 0.2280 0.2738 C10 0.0748 0.0755 C21 –0.0252 –0.0013 C2 0.2840 0.2743 C11 0.0005 –0.0255 C22 0.3908 3888 N3 –0.3560 –0.4968 C12 0.3849 0.3914 C23 –0.0307 –0.0457 N4 –0.1025 –0.4976 C13 –0.0592 –0.0318 C24 0.0965 0.0910 C5 0.2662 0.2395 C14 0.4566 0.3905 O25 –0.5188 –0.6785 C6 0.2649 0.2378 O17 –0.5356 –0.5349 O27 –0.5284 –0.5285 O8 –0.6465 –0.6820 C19 –0.0832 –0.0900 Pd … 1.2309 C9 –0.0841 –0.0909 C20 0.4363 0.3803 The values of some thermodynamic parameters (total energy, zero-point energy, EHOMO, ELUMO, etc.) of the palladium complex are available from the calculations For example, the zero-point vibrational energy, calculated by the DZ basis set, is 7.342860 eV The HOMO energy obtained at PW91/TZP level is higher than DZ and DZP They are –4.688, –5.291, and –5.135 eV, respectively The dipole moment obtained at the PW91/DZ level is the highest one (8.27943199 D), and the values obtained at the PW91/DZP and PW91/TZP levels are 6.29644665 and 6.13886843 D, respectively Finally, the HOMO/LUMO gaps obtained from both PW91/DZP and PW91/TZP calculations are 1.939 and 1.930 eV, respectively The H L Schiff-base ligand and its Pd(II) complex were characterized by elemental analysis, spectroscopic data (IR, UV-Vis), and molar conductance The thermogravimetric analysis (TGA) of palladium complex was done as well The electronic spectra of the ligand and complex were recorded in DMSO (see Figures and 4) The UV-Vis spectrum of the free ligand H L exhibits absorption bands, at 280, 308, and 376 nm The absorption at 280 nm can be assigned to the π – π * transition of the benzene rings A low-intensity band in the lower energy region at 308 nm is attributable to a π –π * transition of the azomethine chromophore The band around 376 nm is due to the n– π * transition of the nonbonding electrons present in the nitrogen of the azomethine group in the Schiff base 38,39 871 AKBARI and ALINIA/Turk J Chem Figure UV spectrum of the ligand, (a) experimental; (b) calculated (PW91/TZP) Main calculated transitions from Table 4: (1) Excitation 6–10, (2) Excitation 2–5, (3) Excitation Figure UV spectrum of the Pd(II) complex, (a) experimental, (b) calculated (PW91/TZP) Main calculated transitions from Table 3: (1) Excitation 8–10, (2) Excitation 5–7, (3) Excitation 2–4, (4) Excitation Compared to the ligand, the π –π * transitions of the benzene rings in the complex are red shifted, and the π –π * transitions of the azomethine are blue shifted to some extent, probably as a result of delocalization of the electrons from the nitrogen to the metal ion as well as destruction of the hydrogen-bonded chelate ring Another important characteristic of the spectrum of the complex compared to the free ligand is the absence of the n – π * band 38 The electronic spectrum of a square-planar complex shows d–d spin allowed transitions These correspond to the transitions from the lower lying ‘d’ levels to the empty d x2 −y2 orbital The ground state is A g and the excited states corresponding to the above transitions are A 2g , B 1g , and E g in order of increasing energy Three d–d bands of a similar complex are reported in every region, 463–523 nm, 405–418 nm, and 344–386 nm, which are attributed to A g →1 A g, A g →1 B g, and A g →1 Eg transitions, respectively 22 The electronic spectrum of the complex showed a band at 437 nm This band could be attributed to mainly ligand-to-metal charge transfer composed of a transition between the ligand (N = C) HOMO to the metal centered LUMO in the partially filled MO The electronic excitation energies and oscillator strengths, f, calculated by the PW91/TZP method for the mentioned ligand and complex are summarized in Tables and 4, respectively Comparisons of the calculated and experimental spectra for the ligand and Pd(II) complex are shown in Figures and 4, respectively Figure shows absorption (see also Table 4) while Figure contains 872 AKBARI and ALINIA/Turk J Chem more The latter (calculated at 433 nm) could be due to d–d transition, which has very low intensity in the experimental spectrum and is covered by the shoulder of 365 nm absorption Table All singlet–singlet calculated excitations for the ligand, using PW91/TZP level Exc Composition E(eV) H H–1 L (91%) H–2 L (40%), H H–2 L+1 (13%), H–3 H–1 L+1 (60%), H L+1 (11%), H H–1 L+1 (30%), H–2 L (21%), H Oscillator theo exp Strength (f) (scaled (nm)) (nm) L+1 (51%), H L+1(15%), H–2 H L (48%) L (15%), H L (10%) H–3 L+1 (47%), H –3 H–4 L (75%), H–4 H–2 L+1 (34%), H–4 10 L (11%) L (15%), L+1 (11%) L+1 (14%), L (37%), H–2 L+1 (10%) L+1 (21%) L+1 (28%), H–2 L (15%), H–3 L+1 (12%) H L+2 (98%) H–3 L (33%), H–3 L+1 (33%), H–4 L+1 (11%) 2.56708 0.008178 348 3.00528 0.000446 297 3.09222 0.026630 289 3.13407 0.007869 284 3.15953 0.015700 282 3.26834 0.022280 273 3.51290 0.008520 253 3.68324 0.052340 242 3.76410 4.01591 0.001623 237 222 376 308 280 Table All singlet–singlet calculated excitations of Pd(II) complex using PW91/TZP method Exc Composition E(eV) Oscillator strength (f) H 2.05896 0.001227 433 MMCT& LMCT H–1 2.45337 0.000122 363 LMCT H L+1 (85%) H–1 L+1 (6%) 2.47363 0.010780 360 ILCT H L+2 (83%) H–1 L+2 (7%) 2.51652 0.015110 354 ILCT H–3 L (97%) 2.82355 0.000229 316 LMCT H–4 L (97%) 2.91498 0.000388 306 LMCT H–1 H–1 H–2 L+1 (39%) L+2 (47%) L (20%) 3.01279 0.031220 295 H–1 H–1 L+2 (39%) L+1 (38%) 3.09235 0.021280 288 H–2 H–2 L+1 (87%) L+2 (5%) 3.14669 0.001364 283 10 H–2 L+2 (82%) 3.17049 L (99%) L (98%) theo a (scaled (nm)) exp (nm) Ass 365 297 ILCT (Major) ILCT MLCT 288 0.000407 281 MLCT H−1 = HOMO−1 , H= HOMO, L=LUMO, L+1 = LUMO +1 etc ILCT= inter-ligand charge transfer; MLCT= metal to ligand charge transfer; LMCT= ligand to metal charge transfer, Exc.= excitation, Ass.= assignment; a Scaling factor = 0.72 873 AKBARI and ALINIA/Turk J Chem In the calculated spectrum of the complex, the band centered around 365 nm appears to be a composition of excitations at 2.45337 eV ( λ = 363 nm, f = 0.1220E–03), 2.47363 eV (λ = 360 nm, f = 0.1078E–01), and 2.51652 eV (λ = 354 nm, f = 0.1511E–01), and are due to HOMO −1 to LUMO, HOMO to LUMO +1 , and HOMO to LUMO +2 transitions, respectively Therefore, this transition has a mixed ILCT and LMCT character The main orbital contribution in the transition at 297 nm, which appears as a shoulder, arises from the HOMO −3 to LUMO, HOMO −4 to LUMO, HOMO −1 to LUMO +1 , HOMO −1 to LUMO +2 , and HOMO −2 to LUMO transition This transition can be described as a super position of the transitions at 2.82355 eV (λ = 316 nm, f = 0.2285E–03), 2.91498 eV (λ = 306 nm, f = 0.3879E–03), and 3.01279 eV (λ = 295 nm, f = 0.3122E–01) Therefore, this transition has a mixed ILCT and LMCT charge transfer character The very close absorption at a little more energy ( λ = 288 nm) also consists of more than one excitation Thus, for the transition at ∼ 288 nm, the charge transfer character is logically assigned to ILCT and MLCT transitions The excitations observed to have the respective energies, 3.09235 eV ( λ = 288 nm, f = 0.1511E–01), 3.14669 eV (λ = 283 nm, f = 0.1364E–02), and 3.17049 eV (λ = 281 nm, f = 0.4069E–03) By comparing the experimental and calculated absorption spectra of the complex, one can notice that both sets of data are in good agreement in terms of band positions The HOMO and LUMO are very important parameters in quantum chemistry They are called the frontier orbitals The frontier orbital gap, the difference between their energy levels, helps us to characterize the chemical reactivity and kinetic stability of the molecule A molecule with a small frontier orbital gap is more polarizable and is generally associated with a high chemical reactivity and low kinetic stability and is also termed as the soft molecule A large frontier orbital gap implies high stability for the molecule in the sense of its lower reactivity in chemical reactions The MO energy diagram in the PdL complex, calculated from the PW91 level and TZP basis set, showed that the energy span between the HOMO and the LUMO is 1.93 eV Figure Isodensity plots of the frontier orbitals of the PdL and H L ligand The detailed contributions from different fragments to the frontier orbital for the complex are given in Table In the case of our Pd complex, the HOMO is mainly derived from 2p z orbitals of carbon atoms, but the LUMO is delocalized on the Pd center The percentage composition of the lowest unoccupied and highest occupied molecular orbital levels for this compound are: for the HOMO level, Pd(%) = 12.81d xz + 1.83d x2 −y2 + 1.57d z2 , O(%) = 2.87p x + 22.28p z , N(%) = 7.42p z , and C(%) = 36.24p z For the LUMO level, Pd(%) = 44.17d xy + 2.1d xz , O(%) = 3.72p x + 12.97p y , N(%) = 4.61s + 10.26p y + 5.1p x , and C(%) = 874 AKBARI and ALINIA/Turk J Chem Table Energies (eV) and composition (%) of frontier molecular orbital’s of the complex Orbital Pd % N% C% O% HOMO–4 1.09dxz 7.11 pz 3.69 px+1.31py+ 52.02 pz 1.01px+18.62pz HOMO–3 12.19dyz+1.86dz2 10.57pz 42.52pz 13.29pz - - 27.62py+1.31pz 2.99 pz 52.01 pz 2.78px+22.33pz HOMO–2 37.45dz +7.34dxz+6.63 dx2y2+3.28dyz+2.58s HOMO–1 1.70 dyz HOMO 12.81 dxz+ 1.83 d x2- y2+ 7.42pz 36.24pz 2.87px+22.28pz LUMO 44.17dxy+2.1dxz 4.61s+10.26py+5.1px - 3.72px +12.97py LUMO+1 1.77dyz 16.01pz+1.77px LUMO+2 LUMO+3 1.37dyz - 1.86 px+2.45py+13.97 pz - 5.14 px+2.74 py+51.33 pz 9.37px+73.37pz 6.67pz 5.66 pz - 10.02 px+6.11py+68.3 pz 5.16 pz 1.57dz2 LUMO+4 79px+56.1pz 7.1pz Selected experimental and calculated IR vibrational frequencies (cm −1 ) of the H L ligand and its Pd(II) complex are listed in Table The differences between the theoretical and experimental frequencies are lower than 2.9% The discrepancies between the results may come from the application of the harmonic approximation for calculations performed in the vacuum Nevertheless, all frequencies were corrected by a scaling factor, 0.9648 The sharp band at 1635 cm −1 in the free H L ligand is assigned to the ν (C = N) mode of the azomethine group The shift of this frequency to lower wave numbers (see Table 6) in the complex indicates involvement of azomethine in coordination 40,41 Table Some selected experimental and calculated IR vibrational frequencies (cm −1 ) of the H L ligand and PdL complex Experimental frequencies Calculated frequencies H 2L PdL Ligand (scaled) 3478 3455 2932 2898 2454 Vibrational assignment Relative error (%)b Pd(II) complex (scaled) a Relative error (%)b 3481 0.03 3556 2.92 (O–H) 2938 0.20 2892 –0.20 (C–H) imin - 2399 –2.24 - 1635 1612 1595 –2.44 1566 –2.85 (C=N)imin 1326 1304 1333 0.52 1307 0.23 ip(O– 1280 1295 1278 0.15 1287 –0.61 (C–O)phenolic 1317 1333 1317 0.0 1312 –1.57 ip 1022 1011 1023 0.09 1004 –0.69 (C–N) - 563 - - 559 –0.71 (Pd–O) - 471 - - 475 0.84 (Pd–Nimin) a (N4–H) H) phenolic (C–H) imin Abbreviations: υ, stretching; δ, bending; ip, in-plane; a Scaling factor = 0.9648, b Relative error = (calc – exp/exp) × 100 875 AKBARI and ALINIA/Turk J Chem The phenolic oxygen stretching vibration that appears at 1280 cm −1 in the Schiff base undergoes a blue shift (1295 cm −1 ) in the metal complex This shift confirms the bonding of the phenolic oxygen with the metal 39,42 The free Schiff base exhibits a broad band at 2454 cm −1 , attributed to hydrogen bonding between the phenol hydrogen and azomethine nitrogen The absence of this band in the spectrum of the covalently anchored complex indicates the loss of the hydrogen bond, as expected for the coordination of the deprotonated phenolic oxygen 39,40,43 In the IR spectra of the ligand and Pd(II) complex, the broad bands in the 3232–3625 cm −1 range are attributed to the ν (O–H) stretching vibration of the phenolic groups 44 In the complexes containing water molecules, the absorptions due to O–H stretch observed at 3750 cm −1 and weaker bands in the region 810–750 and 730–700 cm −1 due to ν (OH) rocking and wagging mode of vibrations, respectively, 22 indicate the presence of water 40,45 The presence of multiple bands at 2850–2935 cm −1 in the free Schiff base and its complex, with slight shifts, suggests the presence of a CH group in both of them The stronger band in the 1400–1600 cm −1 range is due to the skeleton stretching vibration of C = C of the benzene ring 46 Thermogravimetric analysis (TGA) was performed to investigate the physical behavior of the complex (see Table 7) Table Thermal data of the PdL complex Mass loss % Complex Temperature (°C) Pd L 2H2O 131–266 266–434 Found Calcd 8.5 65 8.2 68 Assignment Residue –2H2O –L PdL PdO The TGA analysis of Pd(II) Schiff base complex showed that thermal decomposing occurred in steps The first step started at 131 ◦ C and finished at 266 ◦ C, corresponding to the loss of uncoordinated water molecule with a mass loss 8.5% (calc 8.2%) The second stage is due to removing all the N O ligand fragment processes within the range 266–434 ◦ C with mass loss 65% (calc 68%) The final decomposition product is the PdO oxide in good agreement with the calculated weight 25.87% (27.63%), (following Scheme) Conclusion In the present study, the palladium(II) complex of 4-((E)-(2-((E)-2,4-dihydroxy benzylidene amino) ethylimino) methyl) benzene-1,3-diol (H L) was prepared and characterized by thermogravimetric studies, elemental analysis, molar conductance, and spectroscopic techniques, and its geometric structure The electronic structure and spectroscopic properties were thoroughly investigated by DFT calculations as well The results showed that the structure of the complex is distorted square-planer in which the H L acts as a dianionic tetradentate ligand in a N, N, O − , O − manner, via the deprotonated phenolic oxygens and the azomethine nitrogens The calculated parameters are in good agreement with the reported results for a similar compound Furthermore, the DFT calculated IR frequencies and electronic spectra are in close agreement with the experimental results Acknowledgments Special thanks to Dr MA Naseri and Dr R Khalifeh for their help We gratefully acknowledge the support of this work by the Payame Noor University Research Council We are also grateful to Dr SY Ebrahimi, Dr M Ahmadi, and Miss S Shariati and all of our other research group members for their useful cooperation 876 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HOMO/LUMO gaps obtained from both PW91/DZP and PW91/TZP calculations are 1.939 and 1.930 eV, respectively The H L Schiff- base ligand and its Pd(II) complex were characterized by elemental analysis,... 7861–7871 Saheb, V.; Sheikhshoaie, I Spectrochim Acta Part A: Molecular and Biomolec Spec 2011, 81, 144–150 Taha, Z A. ; Ajlouni, A M.; AlMomani, W.; Al-Ghzawi, A A Spectrochim Acta Part A 2011,