Oxovanadium(IV) complex of 8-hydroxy quinoline and 3-acetyl-6-methyl-2H-pyran-2,4(3H)-dione: Experimental, theoretical and antibacterial evaluation

2.1 Materials and methods

Vanadium sulphate pentahydrate (Aldrich chemical Co. USA), 3-acetyl-6-methyl-2H-pyran-2,4(3H)-dione (dhaH) (Merck Specialties Pvt., Ltd. Bombay), 8-Hydroxy quinoline, (Merck) 2-picolinic acid and DMF (Thomas Baker (Chemical) Ltd. Mumbai), ethanol (Bengal Chemical and Pharmaceuticals Ltd.) were used as received. All the chemicals were of analytical reagent grade.

2.2 Analysis

Elemental composition (CHN) was determined micro-analytically at S.A.I.F., I.I.T. Powai, Mumbai. The vanadium content was determined by following a slightly modified procedure reported elsewhere (Maurya et al., 2015). 50 mg weight of the sample placed in a silica crucible was decomposed by mixing in 2–3 mL of concentrated HNO₃ and heating. An orange colored solid (V₂O₅) which was obtained was allowed to get completely dissolved in a minimum amount of dilute H₂SO₄, and the resulting solution was diluted to 100 mL by adding distilled water. Volumetrica estimation of vanadium was finally done using 0.02 M KMnO₄ solution as an oxidizing agent in the presence of sulfurous acid (H₂SO₃).

THERMO Finnigan LCQ Advantage max ion trap mass spectrometer, Bruker alpha-T model FT-IR spectrophotometer and Varian Cary 5000 double beam UV–Vis spectrophotometer were used to record mass, IR and electronic spectra, respectively. Cyclic voltammetry was carried out using tetra butyl ammonium perchlorate (TBAP) as a supporting electrolyte on epsilon BASI. Magnetic, conductance and decomposition records were made on SHERWOOD Scientific UK, Toshniwal Conductivity Bridge and melting point apparatus (Kumar Industries, Mumbai), respectively. Thermogravimetric analysis and EPR spectrum were recorded at SAIF, IIT Mumbai.

Theoretical aspects of the complex was studied using density functional theory (DFT) level under B3LYP/6-311+G specified for nonmetallic atoms and B3LYP/LANL2DZ for vanadium atom. The Gauss View 5.0, a graphical interface associated with Gaussian program was carefully visualized to identify the related vibrational modes for the respective IR calculations (GaussView 5.0). After obtaining zero imaginary frequency in the respective FT-IR simulation it is ascertained that the optimized geometry is a local minima. In addition to frequency calculations, various charge density plots confined to different theories were also evaluated to deepen the theoretical speculation of chemical descriptors. The energies of the various molecular orbitals, especially highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels along with the determination of intramolecular charge transfer (ICT) represents the theoretical route adopted in the investigation (Frisch et al., 2010).

2.3 Preparation of mixed-ligand complex

0.001 M dehydroacetic acid (0.168 g) dissolved in 10 mL of ethanol was mixed with 10 mL ethanolic solution of 0.001 M 8-hydroxy quinoline, (0.145 g). The resulting binary solution was slowly added to the solution of 0.001 M VOSO₄.5H₂O (0.253 g) in 10 mL of ethanol-water (1:1) under constant stirring at 50 °C. A coloured solid began to start precipitating slowly after the ternary mixture was allowed to reflux for 2–3 h, which was finally suction filtered, repeatedly washed with 1:1 ethanol-water and dried in vaccuo.

3.1 Infrared spectral studies

The FTIR analysis of the complex shows that the complex exhibits a strong band in the range 640–655 cm⁻¹, which can be assigned to V—O. The ν(V⚌O) is found at 974–975 cm⁻¹ suggesting the absence of V⚌O⋯V⚌O unit (Maurya et al., 2001). The broad band located at 3418 cm⁻¹ in the IR spectrum of dhaH ligand which is due to hydrogen bonded ν(OH), and a medium band centered at 1236 cm⁻¹ representing ν(C—O) (enolic) suggests the enolic form in solid state. The band due to the ν(OH) of the ligands disappears in the respective complex after deprotonation. The appearance of strong band in the range 1644–1662 cm⁻¹ may be assigned to ν(C⚌N) (Mir et al., 2016). Moreover, the appearance of a strong band at ∼1106 cm⁻¹ in the complex suggests the coordination of the hydroxyl oxygen of the ligand. These results support the bidentate coordination of 8-hydroxyquinolene. The complex also shows a broad band vibrational peak at 3441 cm⁻¹ assignable to ν(OH) mode due to the coordinated H₂O (Maurya et al., 2015). The respective IR spectrum is given in Fig. 1. Fig. 2 is the theoretical FTIR spectrum of the complex. On comparing the results discussed in the experimental section of this sort, it can be established that the DFT-based infra-red spectral data is in excellent agreement with the results being reported.

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Fig. 1

Experimental FT-IR spectrum of [VO(dha)(8-hq)(H₂O)]

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Fig. 2

Theoretical FT-IR spectrum of [VO(dha)(8-hq)(H₂O)]

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3.2 Magnetic measurements and conductance measurements

The observed value of the magnetic moment of the complex is 1.72B.M., as expected for magnetically dilute oxovanadium(IV) complexes. Similarly, experimentally determined molar conductance measured in 10⁻³ M dimethylformamide (DMF) solution of the complex gestures the non-electrolytic behaviour (Popova et al., 2015).

3.3 Mass spectrometry

The electron spray ionization (ESI) (Fig. S1) mass spectral analysis was carried out to confirm the molecular formula of the complex. The representative compound [VO(dha)(8-hq)(H₂O)] gives spectral peaks at 396, 168, 144 m/z indicating [molecular ion]⁺, (396), [dha + H]⁺ = 169 and [8hq + H]⁺ = 144. The m/z 195 can be because of oxovanadium binded cation with quinoline moiety (devoid of hydroxyl oxygen). The results are in good agreement with the proposed molecular composition of the complex.

3.4 Electronic spectral studies

Fig. 3 shows the electronic spectral record of the representative complex [VO(8-hq)(dha)H₂O] carried out in 10⁻³ M DMSO solution. The high intensity peak(s) in the region 263 nm are attributed to intra ligand n → π^∗/π → π^∗ type transitions. The lower intensity peak(s) around 402 nm may be assigned to ligand to metal charge transfer (LMCT) transitions (Maurya et al., 2002). The theoretically generated electronic spectrum (Fig. 4) using the TD-DFT method and DMSO as the solvent at B3LYP/LANL2DZ level of theory shows mainly six bands at 473, 482, 489, 613 and 652 nm. The overall analysis shows close agreement only confined to 400 nm region. The reason of poor agreement can be the less reliability of the theory used for the computation.

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Fig. 3

Experimental electronic spectrum of complex [VO(8-hq)(dha)H₂O]

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Fig. 4

TD-DFT Theoretical electronic spectrum of complex [VO(8-hq)(dha)H₂O]

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3.5 Thermogravimetric analysis

Thermogravimetric curve of the compound, [VO(dha)(8-hq)(H₂O)] is given in Fig. S2. The analysis was carried out in the temperature range of 20–1000 °C at the heating rate of 10 °C/min. Despite the fact that the complex did not show a usual decomposition pattern as expected for the complexes under question. But it can be stated that simultaneous mass losses can also happen under thermal analysis. This unexpected behaviour can be correlated with the similar type of results reported elsewhere (Diaz et al., 2015; Valenzuela et al., 2013). The weight loss of 48.77% at 25–370 °C, can be corresponded to the removal of one molecule of coordinated water (Dodd et al., 1987) and one molecule of dehydroacetic acid anion per molecule of the complex (calculated weight loss = 46.71%). The further raise of temperature under the same heating rate shows two more weight losses, whose correlation could not be made separately. However, the mass degradation in the temperature range 370–820 °C (observed 22.51%) (calcd. 27.7%, can be considered the loss for 1 mol of 8-hq ligand roughly estimated to the removal of one coordinated group. The final residue confined to an invariable weight (observed 28.88%) over 820 °C roughly corresponds to V₂O₅ (calcd. 25.59%).

3.6 Electron paramagnetic resonance studies

From the EPR Spectrum of the complex shown in Fig. 5, the confirmation of unpaired electron in the vicinity of I = 7/2 of its own mother nucleus gives (2 × 7/2 + 1) 8 line spectrum. The observed spectral parameters for this compound are: g⊥ 2.030, g|| 2.0806, g_av = 2.046. The observed g_av value deviates from the free ion value 2.0036, indicates that the complex is appreciably covalent. The respective data has been tabulated in Table 5. The g|| and g⊥ values in the complex ensues the respective covalent nature (Dutta and Syamal, 1993). The features of spectrum indicates monomeric oxovanadium(IV) complex (Maurya et al., 2002).

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Fig. 5

ESR spectrum of complex [VO(dha)(8-hq))(H₂O)] (1).

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3.7 Electrochemical properties

To understand the spectral and structural changes that occur on redox activity of the complex, cyclic voltammetry was invoked. The analysis was carried out in the DMSO vs Ag/AgCl reference electrode at scan rate of 100 mVs⁻¹ under the potential range of -1500 to + 1500 mV in the co-existence of 0.1 M of tetrabutylammonium perchlorate (TBAP) as a supporting electrolyte. The results have shown that the vanadyl complex exhibits one electron quasi-reversible transfer process with a reduction peak at E_pc = −447 to −471 and corresponding oxidation peak at E_pa = −295 to −322 (Fig. S3). Overall the complex shows a predominant reduction. The respective redox couples with peak gap values (ΔE_p) ranging from 125 to 175 mV, clarifies the presence of a single step one electron transfer process. The negative E_r potentials (−383 to −384 mV) may be assigned to the corresponding V(IV) to V(III) conversion (Dai et al., 1996; Maurya and Rajput, 2004). ΔE_p value can be seen showing a increasing trend with the increasing in scan rates, supporting the evidence for the quasi-reversible V(IV)/V(III) couple.

4.1 Geometrical parameters

Geometry optimization is the first and foremost computational process that is generally established to get insights regarding atomic projections to attain energy minimal state of the complex. The z-matrix presentation of different geometry elements of the optimized structure of the complex, [VO(dha)(8-hq)(H₂O)] are given in the Table 1. The optimized structure of the compound (1) is shown in Fig. 6. The computed bond lengths, such as, V⚌O (oxo) (14), V—O (enolic) (11), V—O (ketonic) (1), V—O (water) (26), V-O(25) and V-N(15), in the present complex are 1.6074, 2.1703, 1.9939, 2.1528, 1.9261 and 2.1108 Å, respectively. These results are comparable to the data reported elsewhere (Greenwood and Earnshaw, 1984). O(1)-V(13)-O(11), (75.0591°), O(1)-V(13)-O(14), (100.9314°), O(1)-V(13)-O(25), (88.3392°), O(1)-V(13)-O(26), (93.4652°), O(11)-V(13)-N(15), (84.5678°), O(11)-V(13)-O(25), (90.2716°), O(11)-V(13)-O(26), (72.0918°), O(14)-V(13)-N(15), (98.2726°), O(14)-V(13)-O(25), (110.0045), O(14)-V(13)-O(26), (87.7144°), O(25)-V(13)-N(15), (80.6722°), N(15)-V(13)-O(26), (91.9625°), O(14)-V(13)-O(11), (159.724), O(25)-V(13)-O(26), (161.535), O(1)-V(13)-N(15), (160.225). From the above geometrical data a suitable cis-octahedral geometry of the complex is suggested.

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Fig. 6

Optimized molecular structure of the complex cis-[VO(dha)(8-hq)(H₂O)] (1).

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4.2 DFT-based molecular orbital analysis

Frontier molecular orbitals are significant to speculate and characterize the molecular chemical stability (Sun et al., 2003). Six important molecular orbitals (MOs) namely, HOMO, LUMO, two orbitals down to HOMO and two orbitals above the LUMO were evaluated out for dehydroacetic acid (dhaH) and the complex. The analysis can be seen in Fig. S4(a) and (b). The electronic configuration can be used to verify diamagnetic nature of dhaH and paramagnetic behaviour of complex. From the overall composition the weakest bond is with H₂O reflected from bond length data as well as from orbital composition. The results are in good agreement as per the other existing scientific reports (Micera and Garribba, 2010). The HOMO-LUMO gap can be further co-related with ionization energy (IE) and electron affinity (IA) by applying Koopmans’s theorem (Mir et al., 2018). Above all, the global reactive descriptors can also be invoked as suggested elsewhere. The absolute electronegativity (χ_abs), absolute hardness (η) of dehydroacetic acid and its complex are represented in Table 2. Other important properties related to the dipole moment and hardness is electrophilicity index (ω), global softness (S) are given in Table 2.

4.3 Hyperpolarizability calculations

Density functional theory by the calculation of the dipole moment (μ), mean polarizability (α) and the total first static hyperpolarizability (β) is useful in predicting material properties of a compound (Manzoor et al., 2016; Islam and Pandith, 2014). The related information is helpful in gaining insights for the possible use in electronic, laser and photoactive devices. The respective data of [VO(dha)(8-hq)(H₂O)] in terms of x, y, z components are given in Table 3. It is here to mention that for the calculated electric dipole moment μ in Debye, isotropic polarizability α in a.u., anisotropy of the polarizability Δα in a.u., all hyperpolarizability (β) components in a.u. are the expressed units presented over here.

4.4 Molecular atomic charges and molecular electrostatic potential (MESP)

NBO and Mulliken population analysis of charges are the two hypothetical ways to explain the related partial charges of constituent atoms of a molecule. In case of the representative complex, [VO(dha)(8-hq)(H₂O)] the results shown by NBO and Mulliken population analysis (Mulliken, 1995) under B3LYP/ LANL2DZ basis set are shown in Table 4. The comparison between the two methods can be easily made by looking at the results. From the data the atoms namely C₂, C₄, C₆, C_8, C₂₃, C₂₄, V₁₃ and all hydrogen bear positive charge both in NBO and Mulliken analyses. The remaining atoms are negatively charged in both the analysis. The overall charge distribution is clearer from Fig. S5. It may be mentioned here that the natural population analysis is always made keeping in view the Pauli’s exclusion principle (Mir et al., 2017a,b).

The molecular electrostatic potential surface of dehydroacetic acid and its vanadium complex are given in Fig. 7 (a) and (b), respectively. As expected, the diagram again predicts the predominance of charged regions within a molecular system. The MESP shows dehydroacetic acid having the carbonyl oxygen and hydroxyl oxygen atoms the most negative potential region, while in the case of the complex the coordination environment of four oxygen, one nitrogen and vanadium (central atom) is the region of most negative potential. The green region in the MESP surfaces represents neutral site between two extremes red and dark blue color or negative and positive charged loci. Table 5

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Fig. 7

Molecular electrostatic potential diagram of (a) dehydroacetic acid and (b) [VO(dha)(8-hq)(H₂O)] with color range along with scale.

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