Synthesis, spectroscopic studies and fungicidal activity of some diorganotin(IV) with 2-[(phenylcarbonyl)amino]propanoato

2.1 Synthesis of 2-[(phenylcarbonyl)amino]propanoic acid

One gram of alanine was dissolved in (25 ml) of 5% NaOH solution in a conical flask. To this mixture, benzoyl chloride (2.25 ml) was added in five portions in (0.5 ml increments) and shaken vigorously until all the chloride reacted, acidified with diluted hydrochloric acid and the crude product was washed with cold ether. Finally, the desired product was recrystallized from ethanol.

2.2 Preparation of complexes

Complexes were synthesized by dissolving the free ligand (2 mmol) in hot toluene and adding the diorganotin salts (1 mmol) to the solution. The solution was refluxed for 6 h with a magnetic stirrer and then cooled and filtered. The filtrate was reduced under vacuum to a small volume and solid was precipitated by the addition of petroleum ether, dried at 60 °C and recrystallized from ethanol.

2.3 Instrumentation

Elemental C, H and N analysis were carried out on a Fison EA 1108 analyzer, the FTIR spectra in the range (4000–370) cm⁻¹ cut were recorded as potassium bromide discs using a Perkin–Elmer spectrophotometer GX, molar conductance measurements were made in anhydrous DMF at 25 °C using Inolop-Cond Level 1 WTW, atomic absorption measurements of the prepared complexes were obtained using Shimadzu 680 cc-flame. The ¹H, ¹³C and ¹¹⁹Sn nuclear magnetic resonance spectra were recorded on a jeol 400 MHz spectrometer, relative to the internal standard tetramethylsilane (TMS). Melting points were determined in open capillary tubes using an electrothermal 9300 digital melting point apparatus.

3.1 Infra-red spectroscopy

The FTIR spectrum of the ligand, shows characteristic stretching absorption bands at 3371 cm⁻¹, 3328 cm⁻¹, 1611 cm⁻¹ and 1332 cm⁻¹ assigned to υ(OH), υ(N–H), υ(COO) asym. and υ(COO) sym. groups, respectively.

The COO stretching vibrations are important to predict the bonding mode of the ligand. According to Lebl et al. the values of Δυ [Δυ = υ asym.(COO)−υ sym.(COO)] can be divided into three groups (Reeves and White, 1983); (a) In compounds where Δυ(COO) > 350 cm⁻¹, the carboxylate group binds in a monodentate fashion. However, other very weak intra- and intermolecular interactions cannot be excluded. (b) When Δυ (COO) < 200 cm⁻¹, the carboxylate groups of these compounds can be considered to be bidentate. (c) In compounds where Δυ (COO) > 200 cm⁻¹ and <350 cm⁻¹ an intermediate state between monodentate and bidentate (anisobidentate) occurs. It has also been suggested that the Δυ (COO) value in the chelating mode is less than the Δυ (COO) in a bridging mode (Shahid et al., 2009). From the preceding discussion it is proposed that the investigated compounds have chelating-type carboxylates. The disappearance of hydrogen from the hydroxyl group on complexation indicates that the complexation is through the oxygen atom. The bands for υ(Sn–C) and υ(Sn–O) are assigned in the range of (531–556) and (443–447) cm⁻¹, respectively. The IR data of the complexes are shown in Table 2. The table lists the stretching frequency (υ) for some of the characteristics groups exhibited by the ligand and complexes.

3.2 Nuclear magnetic resonance

The ¹H NMR spectra for all compounds were recorded in [²H₆] DMSO using tetramethysilane as the internal standard. The data are compiled in Table 3. The conclusion drawn from ¹H NMR studies of a few compounds lend further support to suggested the formation of 2-[(phenylcarbonyl)amino]propanoic acid chelate. Ligand (HL) gives a single resonance near δ 8.71 ppm attributable to the N–H proton. The spectra also exhibit a singlet –OH peak at 9.24 ppm due to hydroxyl group. The hydroxyl resonance is absent in the spectra of the complexes indicting deprotonation and coordination of Tin to the oxygen. There is a small upfield shift of the aromatic protons resonances of the ligand upon chelation with the diorganotin(IV) moiety (Yousif et al., 2009a). The complexes Ph₂SnL₂, Bu₂SnL₂ and Me₂SnL₂ show additional signals. The methyltin (Sn-CH₃) accurse at 1.35, 1.33 and 1.31 ppm as on the sharp singlet at integrates for the protons accompanied by satellites due to the ¹H–¹¹⁹Sn coupling that corresponds to the hydrogen atom of the methyl protons for the Me₂SnL₂. In dibutyltin(IV) complex the butyl protons appears as a multiplet and a triplet in the range of 1.55–0.72 ppm due –CH₂CH₂CH₂CH₃ group. The aromatic protons in Ph-Sn appears in the 7.06–7.17 ppm.

Table 4 shows the most relevant ¹³C and ¹¹⁹Sn NMR data. Due to scant solubility of the ligand and its complexes in the CDCL₃, their spectra were recorded in [²H₆] DMSO. The C⚌O resonance group of the complexes at (160.22–160.31) ppm where shifted downfield compared with the position in the free ligand, which appeared at 165.32 ppm. It is most likely that shift is due to the decrease of electron density at carbon atoms when oxygen is bonded to the metal ion. This observation lends further evidence that the complexation occurred through the oxygen atoms of the carboxylate group. ¹¹⁹Sn NMR spectra for the complexes were recorded in [²H₆] DMSO. Diorganotin(IV) complexes gave resonance at −442.86, −436.83 and −431.49 ppm related to Ph₂SnL₂, Bu₂SnL₂ and Me₂SnL₂, respectively which is well within the range for six-coordinated complexes. In Ph₂SnL₂ the ¹¹⁹Sn resonance appears, as usual, at a lower field region than in Bu₂SnL₂ and Me₂SnL₂ in spite of the greater electron withdrawing capability of the phenyl group. The resonance at (−442.86 ppm), probably reflects the greater shielding ability of the phenyl group.

On the basis of the preceding discussion, the structure of the complexes suggested as follows:

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3.3 Biological activity

On the other hand, the newly prepared compounds were screened for their in vitro antifungal activity against Gibberela, Cercospora arachidicola, Physolospora piricola and Fusarium oxysporum in DMF by the serial plate dilution method (Reeves and White, 1983). All the fungal strains were clinical isolates, identified with conventional morphological and biochemical methods. Sabouraud’s agar media was prepared by dissolving peptone (1 g), d-glucose (4 g), and agar (2 g) in distilled water (100 ml) and adjusting the pH to 5.7. Normal saline was used to make a suspension of the spore of fungal strain for lawning. A loopful of a particular fungal strain was transferred to 3 ml saline to get a suspension of the corresponding species. Agar media (20 ml) was poured into each petri dish. Excess suspension was decanted and the plates were dried by placing in an incubator at 37 °C for 1 h. Antifungal activity was determined by measuring the diameter of the inhibition zone. The zone of inhibition observed after respective incubation was measured and percent inhibition of the compounds were calculated. The results are presented in the Table 5 and show that all compounds display certain activity to P. piricola at a low concentration. Moreover, the Ph₂SnL₂ is more active than the other diorganotin derivatives. In addition, Ph₂SnL₂ shows the highest inhibition percentage for P. piricola (80.7%) in vitro. (Table 5).