Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 136 (2015) 625–634 Contents lists available at ScienceDirect Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa Structural investigation of oxovanadium(IV) Schiff base complexes: X-ray crystallography, electrochemistry and kinetic of thermal decomposition Mozaffar Asadi a,⇑, Zahra Asadi a, Nooshin Savaripoor a, Michal Dusek b, Vaclav Eigner b,c, Mohammad Ranjkesh Shorkaei a, Moslem Sedaghat a a Department of Chemistry, College of Science, Shiraz University, Shiraz 71454, Iran Institute of Physics AS CR, v.v.i., Na Slovance 2, 182 21 Prague, Czech Republic c Department of Solid State Chemistry, Institute of Chemical Technology, 166 28 Prague, Czech Republic b 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  Kinetic of thermal decomposition showed that the complexes had good thermal stability.  Electrochemical studies showed the difference in redox potential of the complexes according to the substitutional groups.  The X-ray of vanadyl Schiff base complex showed two different crystal structure. a r t i c l e i n f o Article history: Received 5 July 2014 Received in revised form 16 September 2014 Accepted 19 September 2014 Available online 23 October 2014 Keywords: Oxovanadium(IV) complexes Schiff base Kinetics of thermal decomposition Electrochemistry a b s t r a c t A series of new VO(IV) complexes of tetradentate N2O2 Schiff base ligands (L1–L4), were synthesized and characterized by FT-IR, UV–vis and elemental analysis. The structure of the complex VOL1DMF was also investigated by X-ray crystallography which revealed a vanadyl center with distorted octahedral coordination where the 2-aza and 2-oxo coordinating sites of the ligand were perpendicular to the ‘‘-yl’’ oxygen. The electrochemical properties of the vanadyl complexes were investigated by cyclic voltammetry. A good correlation was observed between the oxidation potentials and the electron withdrawing character of the substituents on the Schiff base ligands, showing the following trend: MeO < H < Br < Cl. We also studied the thermodynamics of formation of the complexes and kinetic aspects of their thermal decomposition. The formation constants with various substituents on the aldehyde ring follow the trend 5-OMe > 5-H > 5-Br > 5-Cl. Furthermore, the kinetic parameters of thermal decomposition were calculated by using the Coats–Redfern equation. According to the Coats–Redfern plots the kinetics of thermal decomposition of studied complexes is of the first-order in all stages, the free energy of activation for each following stage is larger than the previous one and the complexes have good thermal stability. The preparation of VOL1DMF yielded also another compound, one kind of vanadium oxide [VO]X, with different habitus of crystals, (platelet instead of prisma) and without L1 ligand, consisting of a V10O28 cage, diaminium moiety and dimethylamonium as a counter ions. Because its crystal structure was also new, we reported it along with the targeted complex. Ó 2014 Elsevier B.V. All rights reserved. ⇑ Corresponding author. Tel.: +98 711 613 7121; fax: +98 711 646 0788. E-mail addresses: asadi@susc.ac.ir, mozaffarasadi@yahoo.com (M. Asadi). http://dx.doi.org/10.1016/j.saa.2014.09.076 1386-1425/Ó 2014 Elsevier B.V. All rights reserved. 626 M. Asadi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 136 (2015) 625–634 Introduction There has been considerable interest in the chemistry of transition metal complexes of Schiff bases, due to the fact that Schiff bases stabilize many different metals in various oxidation states [1]. Vanadium is actually known as a trace element essential for higher organisms. The coordination chemistry of vanadium is of great interest because of its presence in abiotic as well as biotic systems [2,3]. Vanadium can be used as a catalyst for various types of reactions [4], it exhibits variety of insulin mimetic properties and plays a role in many enzymatic reactions. The biochemical aspects of vanadium complexes are the driving force for research of the coordination chemistry of vanadium [5]. Keeping all these facts in mind, we present here the synthesis and characterization of the ligands obtained from the reaction of salicylaldehyde derivatives with 2-aminobenzylamine and their vanadium complexes. We also report the electronic effect of salicylaldehyde derivatives in the ligands on the thermodynamics, thermal and electrochemical properties of their vanadyl(IV) Schiff base complexes. The kinetic parameters of thermal decomposition, calculated using the Coats–Redfern method are also presented. Experimental All chemicals and solvents used for synthesis and electrochemistry were of commercially available reagent grade and used without purification. Scanning UV–vis measurements were carried out on a Perkin-Elmer Lambda 2 UV–vis spectrophotometer equipped with a LAUDA ecoline RE 104 thermostat. The 1H NMR (250 MHz, CDCl3 or DMSO-d6, TMS) spectra were recorded on Bruker Avance DPX 250 MHz spectrometer. IR spectra were recorded on Shimadzu FT-IR 8300 infrared spectrophotometer. Elemental microanalyses (C.H.N.) were obtained using a CHN Thermo-Finnigan Flash EA1112. BUCHI 535 instrument was used to obtain the melting point of the compounds. Thermogravimetric measurements were performed on a Perkin-Elmer Pyris Diamond Model. Electrochemistry studies were recorded using Auto lab 302N. X-ray single-crystal diffraction experiment was performed on four-circle diffractometer Gemini of Agilent Technologies with kappa geometry, equipped with a Copper sealed tube, Cu-Ultra collimator with mirrors and CCD detector Atlas. The diffraction data were processed with Crysalis Pro [6], the structures were solved with Superflip [7], refined with Jana2006 [8] and plotted by Diamond 3 of crystal impact. Hydrogen atoms attached to carbon atoms were kept in theoretical positions, those attached to nitrogen atoms were refined freely. The cyclic voltammetry experiments were carried out with a three electrode apparatus. The working electrode was a glassy carbon disc, polished with an Al2O3 suspension prior to every experiment. Ag/AgCl and Pt foil were used as reference and counter electrodes, respectively. The solutions of complexes (1.0  103 mol L1) in CH3CN, and tetrabutylammuniumperchlorate (0.1 mol L1) as a supporting electrode were prepared. All compounds were investigated at 25 °C and the voltammograms were recorded with a potential scan of 100 mV s1. The measurements of formation constant were done using UV–vis absorption spectroscopy through titration of the ligands with various concentrations of metal ions at constant ionic strength (0.10 M NaClO4) and at 25.0 (±0.1 °C). The interaction of NaClO4 with a ligand and the metal ions in methanol was negligible. In a typical measurement 2.5 ml of the ligand solution was transferred into thermodynamic cell compartment of UV–vis instrument and titrated by the metal ion solution. The titration was performed with aliquots of the metal ion with Hamilton 50 ll syringe to the ligand. The absorption measurements were carried out at various wavelengths where the difference in absorption was the maximum after equilibrium. The final spectra of products show different absorption bands from the free ligands, while the metal ion solutions show no absorption at any wavelength. Synthesis of the ligands The tetradentate Schiff base ligands, L1–L4, were prepared according to the literature [9] by condensing a hot solution of 1 mmol of 2-aminobenzylamine with a hot solution of 2 mmol of salicylaldehyde and its derivatives in methanol and refluxing for 3 h. The pure yellow solid was filtered, washed with cold Et2O(5 ml), dried in vacuum and used without further purification. Synthesis of the complexes A methanolic solution of VO(acac)2 (1.0 mmol) was added to 30 ml chloroform solution containing 1.0 mmol of the ligand. The solution was refluxed for 2 h. The precipitate was filtered and washed with chloroform (5 ml) and Et2O (5 ml) (Scheme 1). Growth of the crystals for X-ray crystallography Single crystals of the vanadyl complex, VOL1DMF, were obtained in good yield from slow diffusion of diethyl ether into a solution of the metal complex in dimethylformamide (DMF) at room temperature. The preparation of VOL1DMF yielded also another compound, [VO]X, with different habitus of crystals (platelet instead of prisma). Although it did not contain the L1 ligand we reported it along with VOL1DMF because its structure was new. We believe that peroxide impurities of diethyl ether oxidized the vanadyl Schiff base complex yielding the cage of vanadium oxide. This cage is very similar to the vanadium (V) oxide (V2O5) and it confirms our idea. Results and discussion Crystal structure of VOL1DMF complex The vanadyl ion is located in a general position of the noncentrosymmetric space group Pn. The presence of the center of symmetry was excluded already during solution of the phase Scheme 1. The structure of Schiff bases and their complexes. M. Asadi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 136 (2015) 625–634 Table 1 Crystallographic data for VOL1DMF complex. Table 2 Selected bond lengths (Å) and angles (°) for VOL1DMF complex; prisma structure. Complex Formula Formula weight Crystal system Space group T (K) a (Å) b (Å) c (Å) a (°) b (°) c (°) V (Å3) Z Dx (g cm3) F (0 0 0) Nref independent (measured) Tmin, Tmax Robs (reflections > 3r) wR2 (all reflections) 627 C24H23N3O4V 468.39 Monoclinic Pn 120 8.0734(5) 10.3653(4) 13.1415(7) 90 106.468(5) 90 1054.61(10) 2 1.475 486.0 1884 [3756] 0.540, 0.658 0.0320(2878) 0.0791(2939) problem by charge flipping, which yielded clearly non-centrosymmetric electron density map, and further confirmed by the refinement. The Flack parameter 0.359(6) suggests presence of an inversion twinning. The ligand L1 coordinates to the vanadyl center in a tetradentate fashion forming an equator while the sixth coordination site is occupied with the solvent molecule. This results in a distorted octahedral geometry where 2-aza and 2-oxo coordinating sites of the ligand are perpendicular to the ‘‘-yl’’ oxygen. The coordination geometry around VO is significantly shifted from planarity with the dihedral angle of 26.64(14) between coordination planes of N9–V1–O1 and N17–V1–O25. The crystal lattice of the complex contains a DMF molecule, which is the solvent used for recrystallization. The V@O bond distance in the vanadyl moiety of the complex is 1.600(2), which is typical value for vanadyl compounds [10,11]. The V1–O1 and V1–O25 bond distances [1.954(2), 1.955(2)] are shorter than V1–N9 and V1–N17 V1AO1v V1AO1 V1AO25 V1AO1s V1AN9 V1AN17 O1sAC2s O25AC24 O1AC2 C18AN17 C8AN9 C16AN17 C10AN9 C18AC19 C7AC8 1.600(2) 1.954(2) 1.955(2) 2.353(2) 2.112(2) 2.087(2) 1.238(5) 1.311(4) 1.319(4) 1.282(4) 1.301(4) 1.485(3) 1.430(3) 1.453(4) 1.436(4) O25AV1AO1 O25AV1AO1v O25AV1AO1s O25AV1AN17 O25AV1AN9 O1AV1AO1v O1AV1AO1s O1AV1AN17 O1AV1AN9 O1vAV1AO1s O1vAV1AN9 O1vAV1AN17 N9AV1AN17 N9AC8AC7 N17AC18AC19 C10AN9AC8 C16AN17AC18 87.76(9) 99.9(1) 86.16(9) 88.51(9) 166.90(9) 105.6(1) 81.74(9) 156.57(9) 88.86(9) 170.6(1) 93.2(1) 97.8(1) 89.57(9) 126.7(3) 125.6(3) 117.0(2) 118.1(2) [2.112(2), 2.087(2)], which indicates stronger coordination of the oxygen atoms. Crystallographic data and details of the data collection are listed in Table 1, a molecule of the complex is shown in Fig. 1. Selected bond parameters are listed in Table 2. Crystal structure of [VO]X The monoclinic structure of [VO]X with space group P21/c consists of V10O28 cage, fragments of diamine moiety and fragments of DMF as the recrystallization solvent (Fig. 2a). The cage consists of five symmetry independent vanadium atoms and fourteen symmetry independent oxygen atoms expanded through the center of symmetry. Each vanadium is surrounded by six atoms of oxygen in distorted octahedral geometry. The V–O bonds pointing out of the cage keep the typical distance for vanadyl around 1.6 Å (see Fig. 2b) while for vanadium V2 located inside the cage the vanadyl oxygen cannot be identified. Oxygen O12 is bonded weakly with V–O distances above 2.1 Å. Selected bond lengths and angles of [VO]X are collected in Table 3. Because the compound was not Fig. 1. Structure of VOL1DMF complex. 628 M. Asadi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 136 (2015) 625–634 Fig. 2. (a) Structure of [VO]X. (b). Distribution of VAO distances in V10O28 cage of [VO]X. Color codes: 1.604–1.611 Å black, 1.683–1.704 Å indigo, 1.812–1.929 Å gray, 1.976– 2.051 Å light yellow, 2.120–2.321 Å white. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) Elemental analysis Table 3 Selected bond lengths and angles for ‘‘VO’’ cage of [VO]X. V1AO2 V1AO3 V1AO5 V1AO11 V1AO12 V1AO14 O2AV1AO3 O2AV1A O5 O2AV1A O11 O2AV1AO12 O2AV1AO14 O3AV1AO5 O3AV1AO11 O3AV1AO12 O3AV1AO14 O5AV1AO11 O5AV1AO12 O5AV1AO14 O11AV1AO12 O11AV1AO14 O12AV1AO14 The elemental analysis (Table 4) is in good agreement with those calculated for the proposed formula. 1.909(2) 1.870(2) 1.821(2) 1.604(2) 2.311(2) 2.059(2) 153.82(8) 91.42(8) 102.0(9) 77.09(7) 82.29(8) 92.37(8) 102.16(9) 77.79(7) 84.15(8) 104.02(9) 82.16(7) 156.95(8) 173.80(8) 98.98(9) 74.83(7) IR characteristics The IR spectra of the free Schiff base ligands and the complexes exhibit several bands in 400–4000 cm1 region (Table 5). As a result of replacing the hydroxyl hydrogen of the Schiff base ligands by the metals, the strong band at about 3417–3448 cm1 disappeared. The bands at 2823–3070 cm1 in the Schiff base ligands and complexes are assigned to aliphatic and aromatic CAH modes of vibrations [12].The stretching vibration of the azomethine group (C@N) in Schiff base ligands is observed in the range 1612– 1635 cm1 [13,14]. In complexes, these bands are shifted to lower frequencies, indicating that the nitrogen atom of the azomethine group is coordinated to the metal ion. Stretching bands in the range 1410–1566 cm1 are due to the skeleton stretching vibration of C@C of the benzene ring [15]. The vanadyl complexes show a band at the range 864–972 cm1 attributed to V@O frequency [16]. the targeted complex, other tables have been deposited as supplementary material. The deposited Table S1 collects the basic crystallographic data for [VO]X. The deposited Table S2 documents geometric similarity of the C8H9N2 cyclic moiety found in [VO]X with the diamine moiety of the ligand in VOL1DMF, which supports the idea that C8H9N2 was separated from the ligand L1. The deposited Table S3 makes similar analysis for C2H8N moiety of [VO]X and the DMF molecule supporting the idea that C2H8N was separated from DMF. Electronic spectra With the aim of obtaining information about the type of the electronic transitions and interactions in solution, the electronic spectra of ligands and their complexes (Fig. 3) were recorded in MeOH (Table 6). The recorded spectra of the ligands have revealed two main absorption bands. The first band observed at long wavelength can be ascribed to the p–p⁄ transitions of azomethine system and the second band at higher energy is attributed to the Table 4 Characteristic and analytical data for the complexes. Compounds Color m.p. (°C) Yield (%) Found (Calculated) C VOL1H2O VOL2H2O VOL3H2O VOL4H2O Green Green Brown Brown >250 >250 >250 >250 71 81 59 55 61.34 58.46 44.33 52.36 H (61.02) (58.36) (44.16) (52.31) 4.30 4.66 2.88 3.36 N (4.39) (4.68) (2.82) (3.34) 7.09 5.90 4.96 5.83 (6.78) (5.92) (4.91) (5.81) M. Asadi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 136 (2015) 625–634 Table 5 Selected IR bands (ˆmax/cm1) of the Schiff base ligands and their vanadyl complexes. Compounds 1 L L2 L3 L4 VOL1H2O VOL2H2O VOL3H2O VOL4H2O ˆO AH 3433 3417 3417 3448 – – – – ˆC AH 3042 3008–2823 3068–2883 3070–2893 3047–2923 2939–2823 3056–2857 3046–2869 ˆC ˆC @N 1633 1635 1635, 1612 1635, 1612 1612 1627, 1596 1612 1612 ˆV @C 1565–1410 1566–1488 1558–1473 1558–1473 1542–1450 1535–1460 1512–1458 1519–1458 @O – – – – 956 972 871 864 1.2 L4 ---- VOL4 1 0.8 0.6 0.4 0.2 0 200 250 300 350 400 450 500 Fig. 3. The electronic spectra of L4 and VOL4 in methanol. Table 6 UV–vis. absorption bands (nm) of the Schiff base ligands and their vanadyl complexes. Compound 1 L (H) L2(OMe) L3 (Br) L4 (Cl) VOL1H2O VOL2H2O VOL3H2O VOL4H2O p–p⁄ (C@C) p–p⁄ (C@N) n–p⁄ 235 235 247 240 255 260 263 260 275 265 266 265 320 350 335 330 370 390 377 375 – – – – p–p⁄ transition of the phenyl rings of the compounds. The band due to the n ? p⁄ transition of the C@N chromophore can be seen for the free ligands at the range 350–400 nm involving the promotion of electron pair on the nitrogen to an antibonding p⁄orbital of imine group. On complexation this band disappeared suggesting the coordination of azomethine nitrogen to the metal ion, as the formation of the metal–nitrogen bond stabilizes the electron pair on the nitrogen atom [17]. Thus addition of metal ion to the ligand 629 solution causes distinguishable changes in the visible absorption spectra of the ligand, suggesting an instantaneous complex formation in solution. DFT or ab initio studies The geometries of all molecules involved in this study were fully optimized by using the DFT method with the B3LYP functional and basis set, 6-311G was used for all kinds of atoms at complexes. All of DFT calculations were performed using the GAUSSIAN 03 program and then the following molecular descriptors were collected: total energy (TE), dipole moment (DM), atomic charge of central atom frontier orbital energies including HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) and the difference between HOMO and LUMO level energies. The optimized stable molecular structures of the ligands and complexes are shown in Fig. 4 and Fig. S1 and their frontier orbitals (HOMO, LUMO) are shown in Figs. 5 and S2. Selected geometrical parameters including bond lengths, bond angles and other parameters are listed in Tables 7–9. Results showed that all of the bond lengths and bond angles are in the normal range. In order to check the validity of the applied method, X-ray diffraction data of VOL1 complex were used to compare the optimized structures of the complex. The agreement between the computed structure by the DFT or ab initio method and X-ray diffraction data was excellent. Fig. 6 compares the calculated absorption spectra of VOL1 and VOL3 complexes with the corresponding recorded spectra of complexes in methanol solvent. As seen, there is relatively good agreement between the theoretical and experimental spectra. Thus we can use the theoretical spectra to confirm the transition character of each band. In Fig. 7 four selected bands have been identified in the theoretical spectrum of VOL1 complex and the related transition of each bands are shown in the molecular orbital diagram. According to Fig. 7 and Table 10 for each band some important transitions have been shown. For example for band 1 transition has been occurred between 104a ? 108a and 104b ? 107b levels. By considering the electron density of this transition (Table 10) it is concluded that this transition is essentially related to the delocalization of the electrons from phenyl rings of the Schiff base to the C@N moiety. Thus this band can be assign to the pring ? p⁄C@N transition. Similarly, for band 2 some important transitions have been shown: 105a ? 109a, 106a ? 109a and 105b ? 108b. By considering the electron density of these transitions, this band can be assign to the noxygen ? p⁄C@N transition and MLCT, but the portion of n ? p⁄ is more important than MLCT. With the same conclusion bands 3 and 4 can be assign to noxygen ? p⁄C@N transition and MLCT but again the portion of n ? p⁄ is more important than MLCT. Fig. 4. The optimized structure of the ligand L1 and its complex. 630 M. Asadi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 136 (2015) 625–634 Fig. 5. Frontier orbitals of the ligand L1 and its complex. Table 9 The computed electronic properties for complexes at B3LYP method. Table 7 Selected bond lengths in Å by theoretical calculation at B3LYP method. VOL1 VAO(3) VAO(4) VAN(1) VAN(2) VAO(39) VAO(37) 1.975 1.979 2.128 2.103 2.350 1.616 VOL4 VAO(3) VAO(4) VAN(1) VAN(2) VAO(39) VAO(37) VOL1 VOL3 VAO(3) VAO(4) VAN(1) VAN(2) VAO(39) VAO(37) 1.978 1.979 2.130 2.106 2.33 1.615 VAO(3) VAO(4) VAN(1) VAN(2) VAO(39) VAO(37) 2165.68 10.25 0.209 0.086 0.123 1.648 VOL4 VOL2 1.978 1.979 2.130 2.106 2.330 1.615 EB3LYP (a.u.) l (Debye) HOMO (a.u.) LUMO (a.u.) HOMO–LUMO gap (a.u.) Metal charge (c) VOL3 1.968 1.976 2.132 2.108 2.358 1.617 EB3LYP (a.u.) l (Debye) HOMO (a.u.) LUMO (a.u.) HOMO–LUMO gap (a.u.) Metal charge (c) EB3LYP (a.u.) l (Debye) HOMO (a.u.) LUMO (a.u.) HOMO–LUMO gap (a.u.) Metal charge (c) 7312.66 10.55 0.214 0.092 0.122 1.649 VOL2 3084.88 10.55 0.215 0.093 0.122 1.649 EB3LYP (a.u.) l (Debye) HOMO (a.u.) LUMO (a.u.) HOMO–LUMO gap (a.u.) Metal charge (c) 2394.70 7.81 0.202 0.087 0.115 1.644 Table 8 Selected bond angle (°) by theoretical calculation at B3LYP method. VOL1 O3AVAO4 O3AVAN1 O4AVA N2 N1AVA N2 VOL3 90.00 87.29 85.71 89.87 VOL4 O3AVAO4 O3AVAN1 O4AVA N2 N1AVA N2 O3AVAO4 O3AVAN1 O4AVA N2 N1AVA N2 90.22 87.31 85.75 90.06 VOL2 90.28 87.31 85.70 90.11 O3AVAO4 O3AVAN1 O4AVA N2 N1AVA N2 90.09 87.34 85.52 89.75 Thermal analysis The thermal decomposition of the complexes was studied to evaluate their thermal stability as can be seen from the TG/DTG curves presented in Fig. 8. The organic part of the complexes may decompose in one or more steps with the possibility of the formation of one or two intermediates. These intermediates may include the metal ion with a part of the Schiff base and may finally decompose to stable metal oxides. VOL1H2O complex decomposes in three steps. The first step (calc. 4.36%, found 5%) occurs at the range of 343–393 °C and is Fig. 6. The experimental (in methanol) and theoretical electronic spectra of VOL1 and VOL3. attributed to the release of H2O molecule. The second step (calc. 21.79%, found 22%) occurs in the range 393–542 °C and is assigned to the elimination of C7H8. The last step is attributed to the loss of the rest of the ligand with the formation of metal oxide. The decomposition of VOL2H2O occurs in three steps. The first mass loss (calc. 3.8%, found 4%) in the range of 312–392 °C is attributed to the dehydration of the coordinated water. The second mass loss (calc. 16.07%, found 15%) can be seen between 392–498 °C corresponding to the elimination of C6H4 group. The third step is assigned to the loss of the rest of the ligand with the formation of metal oxide. By considering the TG percentage the metal oxide is. 631 M. Asadi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 136 (2015) 625–634 band2 band1 band3 band4 5 2 5 5 0 Molecular Orbitals Molecular Orbitals Molecular Orbitals 0.5 0.5 0.5 0 0 0 0 -0.5 -0.5 -0.5 -0.5 -1 -1 -1 -1 -1.5 -1.5 -1.5 -1.5 -2 -2 -2 -2 -2.5 -2.5 -2.5 -2.5 -4 -4.5 -5 -5.5 -3 -3.5 -4 -4.5 -5 -5.5 -3 Energy (eV) -3.5 Energy (eV) -3 Energy (eV) -3 Energy (eV) Molecular Orbitals 0.5 -3.5 -4 -4.5 -5 -5.5 -3.5 -4 -4.5 -5 -5.5 -6 -6 -6 -6 -6.5 -6.5 -6.5 -6.5 -7 -7 -7 -7 -7.5 -7.5 -7.5 -7.5 -8 -8 -8 -8 -8.5 -8.5 -8.5 -8.5 band1 band2 band3 band4 Fig. 7. Theoretical spectrum of VOL1 complex and the related transition of each bands. Table 10 The electron density of different transitions. Band 1 Ring 1 (%) Ring 2 (%) C@N (%) V (%) 104a 108a 104b 107b 34 27 33 29 43 17 44 17 17 42 17 44 1 4 1 2 Band 2 O(Schiff C@N (%) V (%) 105a 109a 106a 109a 105b 108b 24 4 28 4 28 3 6 45 8 45 8 46 24 4 3 4 1 3 Band 3 107a 109a 106a 109a 106b 107b 19 4 28 3 28 1 7 45 8 42 8 44 38 4 3 4 1 2 Band 4 107a 109a 106a 108a 106b 107b 19 4 28 3 28 4 base) (%) 7 45 8 42 8 44 38 4 3 4 1 2 For VOL3H2O, a mass loss (calc. 3.1%, found 2.5%) occurred within the temperature range 293–370 °C corresponding to the loss of the coordinated water molecule. At the temperature range 370–595 °C a mass loss (calc. 64.4%, found 62%) occurred due to the elimination of a C14H8O2Br2 group. The third step of the thermal decomposition was assigned to the loss of rest of the organic part along with the metal oxide. The weight loss of VOL4H2O takes place in two steps. The first step (calc. 36%, found 39%) occurs between 335–447 °C due to the loss of the coordinated water with an organic part including C8H6NOCl. The second step was assigned to the loss of the rest of the ligand along with the metal oxide. Kinetic aspects The kinetic parameters of decomposition of the complexes (the activation energy Ea and the pre-exponential factor A#) were calculated using the Coats–Redfern Eq. (1) [18]: log    logð1  aÞ T 2 ¼ log   AR 2RT E  1 bE E 2:303RT ð1Þ ðw0 wt Þ where a ¼ ðw , w0 is the initial mass of the sample, wt is the mass 0 wf Þ of the sample at the temperature T, wf is the final mass at the temperature at which the mass loss is approximately unchanged, b is the heating rate and R is the gas constant. In the present case, a plot of left hand side (L.H.S.) of this equation against 1/T gives a straight 632 M. Asadi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 136 (2015) 625–634 Fig. 8. TG/DTA spectra of VOL2H2O. (1) According to the Coats–Redfern plots the kinetics of thermal decomposition of studied complexes is of the first-order in all stages. (2) For all the complexes, free energy of activation for each stage is larger than that for previous one. This is probably due to the unstable intermediate of the proceeding stages. (3) The values of Ea > 10 show that all complexes have good thermal stability. The various kinetic parameters calculated are given in Table 11. Electrochemical measurements 2 Fig. 9. Coats–Redfern plot of VOL H2O complex, step 2 (418–498 °C). line (Fig. 9), which slopes and intercept are used to calculate the kinetics parameters by the least square method. The goodness of fit was checked by calculating the correlation coefficient. The other systems and their steps show the same trend. The entropy of activation S# was calculated using Eq. (2): A¼ KT s S# =R e h ð2Þ where K, h and Ts are Boltzmann constant, Planck constant and the peak temperature, respectively. The enthalpy H# and free energy of activation G# were calculated using Eqs. (3) and (4): Ea ¼ H# þ RT ð3Þ G# ¼ H#  TS# ð4Þ By comparing the kinetic parameters of all complexes, the following results can be obtained: A typical cyclic voltammogram of VOL2H2O complex is shown in Fig. 10. An oxidation peak is observed at about 0.797 V. VOL2H2O is oxidized to the mono cation [VOL2H2O]+. The electron is removed from the nonbonding orbitals and the V(V) complex is formed. Upon reversal of the scan direction, the V(V) complex is reduced to V(IV) at lower potentials. Multiple scans resulted in nearly identical cyclic voltammograms, thereby showing that the five coordinate geometry is stable in both oxidation states, at least on the cyclic voltammetry time scale. These results revealed that the redox process of all vanadyl Schiffbase complexes is the one-electron transfer reaction. The oxidation potentials for the different complexes are set out in Table 12. The formal potentials (E1/2(IV M V)) for the V(IV/V) redox couple were calculated as the average of the cathodic (Epc) and anodic (Epa) peak potentials of this process. In order to investigate the effect of functional groups of the Schiff base ligands on the oxidation potential, a series of the vanadyl Schiff base complexes were studied by the cyclic voltammetry method. The results show that the anodic peak potential (Epa) varies as it can be expected from the electronic effects of the substituents at position 5. Thus, Epa becomes more positive showing the following trend: MeO < H < Br < Cl. The strong electron-withdrawing effects stabilize the lower oxidation state while the electron donating groups have a reverse effect [19]. 633 M. Asadi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 136 (2015) 625–634 Table 11 Thermal and kinetic parameters of decomposition for vanadyl complexes. Also note that the tables are renumbered sequentially. Compounds E# (kJ mol1) DT (°C) S# (J mol1 K1) H# (kJ mol1) G# (kJ mol1) 4.3  10 738.77 226.21 105.05 197.79 210.56 88.21 31.43 10.13 38.45 96.58 170.56 140.88 36.30 27.94 4.6  1011 669.73 162.62 27.81 198.36 213.28 135.64 30.16 19.27 10.08 92.36 164.51 153.48 76.71 19.77 7.4  1012 5.3  105 504.93 4.73 143.42 203.66 148.23 70.106 11.03 1.84 74.78 158.87 1 VOL H2O 348–393 430–542 600–925 93.51 37.76 19.13 VOL2H2O 312–392 418–498 640–900 VOL3H2O 293–370 475–595 650–900 A# (s1) 7 Thermodynamic interpretations The average formation constant of the complexes were calculated in the selected range of spectra by using the SQUAD 84 program [20], designed to calculate values for the formation constants of the proposed reaction model (Eq. (5)), by employing a non-linear, least-squares approach. H2 L þ VOðacacÞ2 $ ½VOL þ 2Hacac Fig. 10. Cyclic voltammogram of VOL2H2O, in acetonitrile at room temperature. Scan rate: 0.1 V/s. ð5Þ The free energy change DG° values of the formed complexes were calculated from DG° = RT ln Kf at 25 °C (Table 13). As an example, the changes in the absorbance spectrum of one ligand (L4) at different molar ratio of added VO(acac)2 in methanol solvent is shown in Fig. 11. The stability of metal complexes with different ligands decreases in sequence: 5-OMe > 5-H > 5-Br > 5-Cl Table 12 Redox potential data of vanadyl complexes in acetonitrile solution. Compounds Epa VOL1H2O VOL2H2O VOL3H2O VOL4H2O 0.766 0.797 0.756 0.746 Epc (IV?V) E1/2 (V?IV) 0.887 0.927 0.877 0.867 0.827 0.862 0.817 0.807 Table 13 The formation constants, log Kf, and the free energy change, DG°, for the complexation of Schiff base ligands with VO2+ in methanol at 25 °C (I = 0.10 NaClO4). Schiff base ligand 1 L L2 L3 L4 Log Kf DG° (kJ mol1) 4.12 8.01 3.19 2.26 23.51 45.70 18.20 12.89 (±0.08) (±0.05) (±0.05) (±0.06) (±0.42) (±0.23) (±0.25) (±0.34) which corresponds to the expected electronic effects of the substituents at positions 5 of Schiff base ligands, i.e. to the order of an increase in both electron-withdrawing and p-acceptor power of the substituents and to the decrease in donor ability of the ligand groups. For example, the 5-OMe substituted ligand acts as a good r-donor because of the high electron releasing power of the OMe groups in the para position in L2 comparing with the nonsubstituted L1 and electron-withdrawing groups in para position (L3, L4). The withdrawing functional groups make the Schiff base a poor donor ligand and decrease the formation constants while the electron donor groups increase the formation constants. Therefore, the ligands having Br and Cl groups have the smallest formation constants while the ligands with OMe group have the highest ones [21,22]. Conclusions In this work a series of new VO(IV) complexes of tetradentate N2O2 Schiff base ligands was synthesized and characterized and subjected to study of thermodynamic, electrochemistry and kinetics. VOL1DMF complex was also studied with single-crystal X-ray analysis. The results, the following conclusions have been drawn: Fig. 11. The electronic spectra of L4 (1.5  105 M) titrated with various concentrations of VO(acac)2 (1.0  103–7.0  102 M) at I = 0.10 M (NaClO4) and at 25 °C in MeOH. (1) X-ray crystallography confirmed formation of the VOL1DMF complex. In the crystalline state it has a non-centrosymmetric structure with one symmetry independent molecule of the complex. Moreover, formation of another compound [VO]X was confirmed which was not a Schiff base complex. (2) By comparing the kinetic parameters of thermal decomposition of the complexes, the following results were obtained:  According to the Coats–Redfern plots the kinetics of thermal decomposition of the complexes is of the first-order in all stages. 634 M. Asadi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 136 (2015) 625–634  The free energy of activation for each following stage is larger than the previous one. This is probably due to the unstable intermediate of the proceeding stages.  The values of activation energy, Ea > 10, show that the complexes have good thermal stability. (3) Electronic factors influence the values of formation constants for the complexes with various substituents on the aldehyde ring. The stability of metal complexes with different ligands decreases in sequence: 5-OMe > 5-H > 5-Br > 5-Cl (4) From analysis of cyclic voltammograms it can be concluded that the anodic peak potential, Epa, for ligands with electron withdrawing substituents is more positive than for ligands with electron donating substituents. This is in agreement with the formation constant, log Kf, trend for these complexes. It can be concluded that compounds with electron withdrawing substituents are not liable to lose or donate their electrons. Acknowledgements We are grateful to the Shiraz university research council for its financial support. The project P204/11/0809 of the Grant agency of the Czech Republic supported the crystallographic part of the work. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.saa.2014.09.076. References [1] M. Bagherzadeh, M. Amini, J. Coord. Chem. 63 (2010) 3849–3858. 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