Keto-enol tautomerism in new silatranes Schiff bases tailed with different substituted salicylic aldehyde

2.1 Materials

1-(3-Aminopropyl)silatrane was prepared according to an already reported procedure (Dumitriu et al., 2012) consisting in treating (3-aminopropyl)triethoxysilane with triethanolamine in 1:1 M ratio, 10 vol% in a mixture of methanol:butanol:toluene (14:1:50 vol ratio), and 3 h stirring at 60 °C. The structure was established by elemental and spectral analyses as well as by X-ray single crystal diffraction.

3,5-Dichlorosalicylaldehyde, 3-methoxysalicylaldehyde, 3,5-di-tert-butylsalicylaldehyde, all of them of 99% purity, were purchased from Sigma Aldrich and used as such. Methanol, butanol, toluene were purchased from Chimopar-Romania, while acetonitrile was acquired from Sigma-Aldrich.

2.2 Equipments

Infrared spectra were recorded using a Bruker Vertex 70 FTIR spectrometer in the transmission mode (KBr pellets) between 4000 and 400 cm⁻¹ at room temperature with a resolution of 2 cm⁻¹ and accumulation of 32 scans.

The NMR spectra were recorded on a Bruker Avance DRX 400 MHz Spectrometer equipped with a 5 mm QNP direct detection probe and z-gradients. Spectra were recorded in DMSO‑d₆ at room temperature.

UV–Vis spectra of the samples 1–3 were registered on a Jenway 6505 Spectrophotometer (Watford, UK) in 10 mm optical path quartz cuvettes by using the chosen solvents (chloroform, methanol, acetonitrile and dimethylformamide) as references.

The carbon, hydrogen, and nitrogen contents were determined by standard methods.

The thermogravimetric analysis was performed on STA 449F1 Jupiter NETZSCH (Germany) equipment. The measurements were made in the temperature range 20–700 °C under a nitrogen flow (50 mL/min) using a heating rate of 10 °C/min.

Differential scanning calorimetry (DSC) measurements were conducted on a DSC 200 F3 Maia device (Netzsch, Germany). A mass of 15 mg of each sample was heated in pierced and sealed aluminum crucibles at a heating rate of 10 °C min⁻¹. Nitrogen atmosphere at a flow rate of 50 mL min⁻¹ was used. The temperature against heat flow was recorded. The device was calibrated using indium according to standard procedures.

Water vapor sorption capacity of the samples have been measured by using the fully automated gravimetric analyzer IGAsorp produced by Hiden Analytical, Warrington (UK).

The study of antimicrobial activity of the new synthesized compounds was carried out using three species of fungi (Aspergillus fumigatus ATCC 66567, Penicillium chrysogenum ATCC 20044, Fusarium ATCC 20327) from pure culture and two bacteria (Pseudomonas sp. ATCC 15780 and Bacillus sp. ATCC 31073) species according to previously reported standard procedures (Zaltariov et al., 2015).

2.3 X-ray crystallography

Crystallographic measurements were carried out with an Oxford-Diffraction XCALIBUR E CCD diffractometer using graphite-monochromated Mo-K_α radiation. The crystals were placed 40 mm from the CCD detector. The unit cell determination and data integration were carried out using the CrysAlis package of Oxford Diffraction (CrysAlis RED, 2003). The structures were solved by direct methods using Olex2 (Dolomanov et al., 2009) software with the SHELXS structure solution program and refined by full-matrix least-squares method on F² with SHELXL-97 (Sheldrick, 2008). Disordered fragments in structure 3 were modeled using available tools of SHELXL97 (PART, DFIX and SADI) in combination with anisotropic/isotropic refinement of non-H atoms.

CCDC 1543444, CCDC 1543446 and CCDC 1543447 contain the supplementary crystallographic data for this contribution. These data obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax:(+44) 1223-336-033; or deposit@ccdc.ca.ac.uk). The main crystallographic data together with refinement details are summarized in Table 1.

2.4 Procedure

3.1 Structural analysis

According to single-crystal X-crystallography, all Schiff bases have a molecular crystal structure built up by the corresponding neutral entities as shown in Figs. 1–3, without any co-crystallized solvent molecules. In all structures, the hydroxyl groups act as donor to the nitrogen atom of the imine group to form an intramolecular H-bond. The hydrogen bonding parameters are shown in Table 2.

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Fig. 1

X-ray molecular structure of compound 1 with atom labeling scheme and thermal ellipsoids at 50% probability level. Non-relevant H-atoms are omitted for clarity.

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Fig. 2

X-ray molecular structure of compound 2 with atom labeling scheme and thermal ellipsoids at 50% probability level. Non-relevant H-atoms are omitted for clarity.

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Fig. 3

X-ray molecular structure of compound 3 with atom labeling scheme and thermal ellipsoids at 50% probability level. Non-relevant H-atoms are omitted for clarity.

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Compound 1 crystallizes in monoclinic system in C2/c space group. The molecular structure of 1 is given in Fig. 1. The molecule 1 is in enol-imine form. The hydrogen atom H4 is located closer to the O4 than to the nitrogen atom N2. The C16—O4 and C10—N2 bond distances are representative for these Schiff bases to determine the enol-imine and keto-amine tautomerism. The values C16—O4 (1.2981(3) Å) and C10—N2 (1.3159(3) Å) are in agreement with enol-imine form (Table 3) of compound 1. In this conformer, the phenolic group hydrogen bonded to the nitrogen atom of the imine group, forming a six-membered ring with O4—H4∙∙∙N2 length 1.81 Å. The bond angles C11—C10—N2 (122.0(3)^o) and C10—N2—C9 (126.2(2)^o) are also consistent with the sp² hybrid character for C10 and N2 atoms (Khalaji et al., 2011).

Compounds 2 and 3 crystallize in monoclinic system in P2₁/c space group. The molecular structures are shown in Figs. 2 and 3. Both structures exhibit enol-imine tautomer form with the intramolecular hydrogen bond O4—H4…N2 at 1.79 Å for 2 and 1.85 Å for 3. The large value of the H-bond is due to the presence of the t-butyl groups attached to the aromatic ring. The bond angles C11—C10—N2 (121.5(5)^o for 2 and 123.7(4)^o for 3) and C10—N2—C9 (121.1(5)^o for 2 and 118.5(4)^o for 3) are also consistent with the sp² hybrid character for C10 and N2 atoms and confirm the enol-imine character of the compounds 2 and 3. The structural characteristics of the compounds 1–3 show a remarkable similarity with that found of another four Schiff bases containing silatrane fragment in the structure (Singh et al., 2013).

The X-ray single-crystal investigation revealed that in all synthesized compounds, the silicon exhibits a pentacoordinated environment with slightly distorted trigonal-bipyramidal geometry.

The formation of Schiff bases was verified by FTIR spectroscopy. The disappearance of the free amine group absorption bands (3294 cm⁻¹ and 3192 cm⁻¹) and presence of the absorption band at 1632–1641 cm⁻¹ is a proof for the azomethine (Gungor et al., 2018; Acar et al., 2018). The other bands characteristics for the silatrane can be found in spectra at about the same wavelength as in 1(3-aminopropyl)silatrane.

¹H NMR and ¹³C NMR spectra also confirm the structure found by X-ray structural analysis (Fig. 1S). In ¹H NMR spectrum can be noticed the presence of the peak at 8.44–8.51 ppm assigned to imine proton, the presence of the peak at 14.17–14.45 ppm assigned to OH proton, besides to those belonging to aromatic protons and the displacement of the protons of the propyl group: from 0.19 to 0.21 ppm (2H, —Si—CH₂—), 1.62–1.66 ppm (2H, Si—CH₂—CH₂—CH₂—NH₂); 3.54–3.60 ppm (2H, Si—CH₂—CH₂—CH₂—NH₂); 2.51–2.78 ppm (6H, N—CH₂—CH₂—O—); 2.78–3.48 (6H, N—CH₂—CH₂—O—).

Aromatic protons from sample 1: 7.36–7.54 ppm (2H).

Aromatic protons from sample 2: 6.7–6.96 ppm (3H) and methoxy protons 3.7 ppm (3H).

Aromatic protons from sample 3: 7.24–7.29 ppm (2H), tert-buthyl protons 1.27 ppm (9H) and 1.39 ppm (9H).

In ¹³C NMR spectrum can be noticed the presence of the peak at 164.98–165.89 ppm assigned to imine carbon.

The presence of the peak at 55.63 ppm assigned to carbon from OCH₃ group for the compound 2 and the peaks at 31.22 (C-18, C-19, C-20) and 29.20(C-22, C-23, C-24) for the carbons from t-butyl group.

3.2 Electronic absorption spectral studies

The absorption spectra of compounds 1–3 in various solvents (chloroform, methanol, acetonitrile and dimethylformamide) are shown in the Fig. 2S–4S and the main spectral characteristics are shown in Table 4.

As it can be seen, the Schiff bases 1–3 exhibited absorption bands in the range 220–300 nm and 320–430 nm, in close relation with the polarity and proton donating capability of the solvents.

In the spectrum of 1, the band at 284 nm in chloroform is assigned to the S₂ ← S₀ (π*, π) transition in benzene ring (Mitra and Tamai, 1999) and shows a hipsochromic shift (4–10 nm) by increasing the solvent polarity. The band at 334 nm is characteristic for the S₁ ← S₀ (π*, π) transition of the enol form and is blueshifted by 10–15 nm in acetonitrile and methanol respectively (Scheme 2). Besides these bands, the spectrum in chloroform, where the intermolecular interactions could be favored (Vargas and Amigo, 2001), revealed one more band at 366 nm with very low intensity, which could be assigned to the extended π-π stacking interactions (Fig. 2S). The band at 428 nm is due to the S₁←S₀ (π*, π) transition of intermolecular hydrogen bonded complexes with the solvent. In methanol, this band shows a hipsochromic shift (10 nm) being of very low intensity and it seems to be due to the enol-keto equilibrium going to the phenol-imine structure (Scheme 2).

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Scheme 2

Molecular structures of 1, 2 or 3 compounds in the corresponding tautomeric species discussed in the text.

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By substitution at the salicylidene ring with methoxy groups in 2, which increase the electronic density on the azomethine bond, the electronic spectra showed a bathochromic shift of the band assigned to the S₂←S₀ (π*, π) transition in benzene ring at 297 nm, as compared with the chloro-substituted derivative 1 (Fig. 3S). The S₁←S₀ (π*, π) transition is present at about 328 nm and is absent in methanol, where the broad band at 416 nm may be put in connection with the stabilization of the S₁ state by intermolecular hydrogen bonds with the solvent, especially with methanol, as compared with the aprotic ones. In methanol, the formation of aggregates is favored by intermolecular H-bonds (Zaltariov et al., 2015).

Its extinction coefficient is larger than 1 and 3 due to increased electron density on the aromatic ring. The same band is slightly red shifted by 4–8 nm in CHCl₃, acetonitrile and DMF.

The electronic spectra of the compound 3 in the four chosen solvents showed the presence of a strong band at 328 nm (CHCl₃) or 326 nm (methanol, acetonitrile and DMF) assigned to the S₁←S₀ (π*, π) transition and a small broad band quite well evidenced in CHCl₃, methanol and DMF at 426 nm, 416 nm and 420 nm, respectively (Fig. 4S). This absorption is due to aggregation of the molecules due to the highly hydrophobic nature of the t-butyl substituents on the aromatic ring. The steric effect of these groups imposes changes in the molecular conformation leading to large separation of the aromatic rings (Zaltariov et al., 2015) and as a result the absence of the S₂←S₀ (π*, π) transition in benzene ring in comparison with 1 and 2.

It can be concluded that substitution with chloro, methoxy and t-Bu groups led to the stabilization in solution of enol tautomer species, depending on the polarity of the chosen solvents. Substitution with methoxy group or the presence of t-Bu substituents led to stabilization only of the enol tautomer in acetonitrile. The position of the absorption maximum of the enol species remains unchanged for 2 and 3, while for 1 has a bathochromic shift (6–9 nm) in CHCl₃ and DMF, due to the different intermolecular interactions between solute and solvents: extended π-π stacking interactions (in CHCl₃) and the larger polarizability of DMF.

The absorption maxima are affected by both solvent polarity and the nature of the substituent (electron donating capability O—CH₃ > Cl > t-Bu). It has been demonstrated the decisive influence of the hypervalent Si←N bond (quite different from a covalent bond) on the physicochemical and spectroscopic characteristics of silatranes (Pedersen et al., 1998; Sidorkin et al., 1980; Singh et al., 2016).

3.3 Dynamic vapor sorption

The behavior of the crystalline Schiff bases in the presence of the humidity was studied by registering sorption/desorption isotherms in dynamic regime. In a special container, the samples were dried at 25 °C in flowing nitrogen (250 mL/min), until their weight was in equilibrium at a relative humidity RH < 1%. After that, the relative humidity was increased step by step from 0 to 90%, in 10% humidity interval, having a pre-established equilibrium time between 3 and 5 min. The sorption equilibrium was obtained for all the steps. Then, the relative humidity was diminished and desorption curves were recorded (Fig. 4).

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Fig. 4

Water vapor sorption-desorption isotherms registered at room temperature.

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According to IUPAC classification, the sorption/desorption isotherms can be associated to type V curves describing sorption on hydrophobic/low hydrophilic material with weak sorbent–water interactions. The maximum sorption capacity value was 2.77 wt% at 25 °C. The desorption process occurs more slowly than sorption and about 0.18–0.22 wt% water is retained in samples 2 and respectively 3.

3.4 Study of Schiff's bases stability in moist environment

As is known, literature data indicate the tendency of the azomethine bond toward hydrolytic cleavage (Bernardo et al., 2006), while the Si—O—C linkage and some silazane compounds tend to be hydrolytically unstable (Noshay and Matzner, 1974). Therefore to be handled properly, it is important to establish how our compounds are sensitive to moisture. The study of the stability of the Schiff base compounds 1–3 during sorption/desorption process was conducted to highlight the effect of water on the azomethine, Si—O and Si—N bonds.

Regarding the azomethine bond, its hydrolytic cleavage can be prevented when the nitrogen atom of the —CH⚌N— group is involved in hydrogen bond formation (Bernardo et al., 2006; Godoy-Alcantar et al., 2005). We thought it would be of interest to examine if the intramolecular H bond between aldehyde O—H group and the nitrogen atom from imine group could improve the resistance of the Schiff bases 1–3 toward hydrolytic cleavage that could take place in the presence of water vapors during the sorption process. Hydration competes with the azomethine formation so that its reversible character is of particular importance and actuality for consideration of the Schiff bases as ideal candidates for the dynamic combinatorial chemistry field (Noshay and Matzner, 1974).

Different methods are used to investigate the hydrolytic stability of the Schiff bases in aqueous media: ¹H NMR technique, UV–Vis, potentiometric measurements, etc.

In this study we used two different techniques: DVS and ATR-FTIR to check the stability of the Schiff bases 1–3, measuring their moisture stability by DVS and the structural changes by IR spectroscopy.

In order to determine the changes that occur in the IR spectra of the samples after their exposure to a relative humidity of 90%, at room temperature for 5 h, ATR-FTIR technique was used.

The spectra were registered using a Bruker Vertex 70 FTIR spectrometer equipped with a ZnSe crystal. The measurements were performed in ATR (Attenuated Total Reflectance) mode in the 4000–600 cm⁻¹ spectral range at room temperature with a resolution of 4 cm⁻¹ and accumulation of 32 scans.

The 3700–2800 cm⁻¹ spectral region of all samples is associated with the O—H groups of the adsorbed water and from aldehyde structure, and C—H from aromatic CH₃ and CH₂ groups (Fig. 5S).

The hydroxyl groups absorb strongly in the region 3300–3600 cm⁻¹. Broad bands are the result of the presence of intermolecular bonding. The precise position of the O—H bands is dependent on the strength of the H bonds and is difficult to establish exactly for these samples the dependence between the adsorbed water molecules and the intensity of the O—H broad band. The intramolecular H bonding (O—H…N⚌CH) also occurs in the case of Schiff bases, the resulting hydroxyl group absorption band being unaffected by concentration changes.

The modes of vibration of aromatic compounds are considered as separate C—H or ring C⚌C vibrations. In general, the bands are of variable intensity and are observed at 3100–3000 cm⁻¹ for ⚌C—H— stretching vibrations.

The asymmetric and symmetric vibrations of the methyl and methylene groups in the structure of the samples 1–3 occur between 3000 and 2800 cm⁻¹. The —CH₃ and —CH₂— asymmetric stretching vibrations occur at 2962 cm⁻¹ and 2924 cm⁻¹ (1), 2973 cm⁻¹ and 2927 cm⁻¹ (2), 2952 cm⁻¹ and 2923 cm⁻¹ (3), while the symmetric one occurs at 2888 cm⁻¹ and 2855 cm⁻¹ (1), 2877 cm⁻¹ and 2837 cm⁻¹ (2), 2876 cm⁻¹ and 2846 cm⁻¹ (3). The position of the —CH₃ and —CH₂— stretching vibration bands is not altered by the exposure of the samples to water vapors (Fig. 5S).

The effect of exposure to water vapors on the azomethine group and Si—N or Si—O bonds of the analyzed compounds were studied by the deconvolution procedure of the IR spectra.

The 1700–1550 cm⁻¹, 1550–1200 cm⁻¹, 1200–950 cm⁻¹ and 950–600 cm⁻¹ spectral regions of the samples 1, 2 and 3 were deconvoluted by a curve-fitting method, and the areas were calculated with a 50% Lorentzian and 50% Gaussian function. The curve-fitting analysis was performed with the OPUS 6.5 software. The procedure led to a best fit of the original curve with an error of less than 0.002.

Fig. 6S shows the 1700–1550 cm⁻¹ spectral region of the samples 1–3 before and after the exposure to water vapors. This region is associated with the —CH⚌N— group at 1636 cm⁻¹ (1), 1634 cm⁻¹ (2) and 1630 cm⁻¹ (3), the ring skeletal vibrations at 1599 and 1576 cm⁻¹ (1), 1608 and 1582 cm⁻¹ (2) and 1613 and 1598 cm⁻¹ (3) and the adsorbed water vibrations after the sorption process at 1662 cm⁻¹ (2) and 1677 cm⁻¹ (3). These data are well correlated with DVS data, where the compounds 2 and 3 retain about 6.8% and 8% water after sorption-desorption processes (Fig. 4).

The IR spectra of the samples in 1550–1200 cm⁻¹ spectral region revealed the presence of the aromatic C—C stretches variable in intensity at 1520–1470 and 1465–1430 cm⁻¹ overlapped with those assigned to O—H deformation vibrations, to aliphatic —CH₂— symmetric bending vibrations at 1430–1350 cm⁻¹ and scissor and deformation vibrations at 1480–1440 cm⁻¹. The —CH₃ asymmetric bending in the case of compound 2 is evidenced by the band at 1440 cm⁻¹, while the t-butyl groups in compound 3 give rise to two bands assigned to the asymmetric —CH₃ deformation vibration at 1474 cm⁻¹ and to —CH₃ symmetric bending vibration at 1354 cm⁻¹. The in-plane O—H deformation vibrations coupled with C—H wagging vibrations give rise to bands in the region 1440–1370 cm⁻¹ which are overlapped with the aliphatic ones. The bands between 1300 and 1200 cm⁻¹ are assigned to alkane C—C skeletal and to C—H in-plane deformation vibrations (Fig. 7S).

The 1200–950 cm⁻¹ spectral region of the samples 1–3 revealed the presence of the asymmetric and symmetric stretches of the Si—O group, as much more intense bands due to the greater ionic character, at 1145 cm⁻¹ and 1101 cm⁻¹ (υ_as) and 1022 cm⁻¹ (υ_s) (1), 1088 cm⁻¹ and 1058 cm⁻¹ (υ_as) and 1020 cm⁻¹(υ_s) (2), 1104 cm⁻¹ and 1089–1050 cm⁻¹ (υ_as) and 1014 cm⁻¹ (υ_s) (3) and asymmetric stretching and —CH₂— wagging (γ_w) vibrations of Si—C bond at 1215 cm⁻¹ (υ_as) and 1192 cm⁻¹ (γ_w) (1), 1174 cm⁻¹ (υ_as) and 1124 cm⁻¹ (γ_w) (2), 1209 cm⁻¹ (υ_as) and 1179 cm⁻¹ (γ_w) (3). The bands at 980–960 cm⁻¹ and 970 cm⁻¹ in the IR spectra of (2) and (3), respectively are characteristic for C—H in-plane deformation vibrations (Fig. 8S) (Socrates, 2001).

The IR spectra of the compounds 1–3 in the 950–600 cm⁻¹ spectral region reveal the presence of the asymmetric and symmetric N—Si stretching vibrations at 859 cm⁻¹ and 795 cm⁻¹ in (1), 878 cm⁻¹ and 804 cm⁻¹ in (2), 880 cm⁻¹ and 809 cm⁻¹ in (3), —CH₂— rocking vibrations between 774 and 680 cm⁻¹, a number of C—H in-plane deformation bands overlapped with O—H out-of-plane deformation vibrations between 940 and 626 cm⁻¹. Beside these, in the IR spectrum of (1) can be seen the characteristic band for C—Cl bond at 683 cm⁻¹ (Fig. 9S) (Singh et al., 2015).

The ATR-FTIR technique was used to evidence if structural changes or chemical interactions occur after the exposure of the Schiff bases to water vapor. IR spectra revealed no structural modification, but only the sorption of water molecules onto the surface of solid samples 2 and 3 as a result of weak physically forces such as van der Waals or H bonds. The data are well correlated with the DVS results, where a slight retention of water of 6.8% and 8% for 2 and 3 respectively was registered. The azomethine bond was not affected by the sorption process mainly due to the intramolecular H bond which stabilizes their hydration cleavage.

No structural changes occurred in the composition of the Schiff bases due to the water vapor sorption, the Si—O and Si—N groups demonstrating a remarkable stability due to the structural packing of them in solid state on the one hand, and on the other hand to the ability of the lone electron pair of the nitrogen atom to interact with the empty orbitals of the silicon atom, that results in a shorter bond distance Si—N which is also an advantage in terms of chemical stability of these compounds.

Based on the results of this study, it can say that in the absence of other factors besides water vapor in the atmosphere of nitrogen, after exposure of 5 h, it is concluded that none of bonds susceptible to hydrolysis, Si—C, Si—N and CH⚌N are not affected.

3.5 Thermogravimetric analysis

Fig. 5A and B show the TGA and DTG curves of the Schiff bases. Characteristic data extracted from these curves are given in Table 5. All three azomethines exhibited three thermal decomposition stages with a maximum weight loss in the range of 261–420 °C. The first stage was assigned to loss of solvent molecules (methanol and acetonitrile) (7.23; 0.91; and 1.48 wt% for compounds 1, 2, and 3). The second step, with initial temperatures between 206 and 255 °C, corresponds to the loss of ligand moiety, debuting with the azomethine linkage scission (—HC⚌N—) (Zaltariov et al., 2014; Kaya et al., 2008). An explanation for the different thermal decomposition pattern of the second stage corresponding to each sample would be due to structural bulkiness, substituents type and to the nature of solvents used in synthesis. The mentioned factors may generate symmetry disruption in the azomethine entity by hindering its structural packing (Al-Ghamdi et al., 2006). The more stable remaining structural entities thermally decompose in the third and last stage, in the range 279–548 °C leaving residue corresponding to the aminopropylsilatranyl moiety. Samples 1 and 2, containing the substituents R₁ = R₂ = —Cl and R₁ = H, R₂ = —OCH₃, left high comparable residue values of 52.48% and 51.01%. The tBu substituent in sample 3 generates more free radicals during thermal decomposition, hence lowering the sample’s thermal stability and residue value (11.63%).

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Fig. 5

TG (A) and DTG curves (B) of the studied structures.

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Fig. 6 shows the DSC curves corresponding to second heating runs of the studied samples.

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Fig. 6

DSC second heating curves of the studied structures.

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As expected, considering their crystalline form, the three compounds show melting transitions at 166 °C (1), 164 °C (2) and 221 °C (3). But, besides this, each of them shows an endothermic transition at lower temperatures, 37 °C (1), 65 °C (2) and 85 °C (3), attributed to the thermochromism phenomenon. It is supposed that a keto-enolic tautomerism occurs in solid state, as was also highlighted in solution in certain solvents (see above), as literature also reports (Zhu et al., 2016; Ogawa et al., 2001). In order to verify this, IR spectra were recorded at transition temperatures marked on DSC curves as T_t and comparatively shown with those recorded at room temperature in Fig. 7. It can see that indeed these transitions correspond to keto-enolic tautomerism.

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Fig. 7

IR spectra revealing keto-enolic tautomerization occurrence at temperatures Tt specific for each sample as identified on DSC curves, compared to those recorded at room temperature.

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3.6 Biological studies (antimicrobial and antifungal studies)

Aminopropylsilatrane and its derivatives are of a special interest as biologically active compounds, with various pharmaceuticals applications as antitumor, antiviral and regulating plant growth. Their anticancer activity has been demonstrated in different cell lines by inhibiting tumor growth through few mechanisms (Ping et al., 2014).

A great importance on the antimicrobial activity of silatrane derivatives has their hydrophobicity mainly due to the alkyl groups favoring on the one hand the hydrolytically stability and on the other hand the direct diffusion through the lipid bilayer due to the lipophilicity (Hearn and Cynamon, 2004).

The antimicrobial efficacy of different biomolecules is dependent on many cellular routes as well as physico-chemical and metabolic characteristics of the various drugs (Fong, 2015). For the Schiff bases, many researches have indicated that their biological activity can be explained based on the structure of the azomethine CH⚌N bond. These compounds possess N, O-donor atoms which can polarize the azomethine group improving its ability for hydrogen bonding and in this way the interaction with the cell membrane of the microorganisms. An important characteristic for the antimicrobial activity of biomolecules is their hydrophobic/hydrophilic balance which can have a large action on the transport across the cell membrane. The most important factors influencing the thermodynamics binding affinity between the drug and membrane transporter are lipophilicity and dipole moment of the drug.

The study of antimicrobial activity of the new synthesized compound was carried out using three species of fungi (Aspergillus fumigatus ATCC 66567, Penicillium chrysogenum ATCC 20044, Fusarium ATCC 20327) from pure culture and two bacteria (Pseudomonas sp. ATCC 15780 and Bacillus sp. ATCC 31073) species according to previously reported standard procedures (Zaltariov et al., 2015; Zaltariov et al., 2013).

The results of the antimicrobial activity as MIC values (µg/ml) for the analyzed compounds were compared with those of the standard ones: Capsofungin (MIC = 0.3 µg/ml) and Kanamycin (MIC = 4 µg/ml). The Schiff bases having chloro and t-Bu substituents, 1 and 3, respectively, have no effect against bacteria and fungi (MIC > 128 µg/ml), while the Schiff base 2 with methoxy substituent on the aromatic ring showed a good antifungal activity (MIC = 0.08 µg/ml).

In the case of the Schiff bases, the antimicrobial activity can be explained based on the structure of the azomethine group, which can be polarized by the presence of different donor atoms N and O. In the case of 2 the presence of the methoxy substituent with electrono-donor properties larger than chloro substituents contributes to the polarization of the imine group increasing its ability for hydrogen bonding with the membrane constituents, indicating their biological activity (Singh et al., 2018).